DISCLAIMER: This is not a piece based on evidence but just personal experience. If you feel there are any factual inaccuracies then please let me know and I'll change them.
I recently got an email asking which of these is a better option for an EFL teacher. Although the person asking probably didn't expect such a long winded reply, it inspired me to put my thoughts down in this blog. I've wanted to write about the DELTA for a while now but this is not that blog...hopefully it will push me to start writing that blog though...so I'm going to lay out some of the pros and cons of both here and let you make up your own mind depending on your needs and situation.
This is fairly straight forward. The DELTA is a 2 month course (with 1 month to write the essay for module 3) and most UK based MAs take a year. things get more complicated if you want to do part time, or distance learning. A lot of people choose the DELTA because taking a whole year off work is quite tough for many EFL teachers -in terms of pay, EFL is hardly banking.
Most DELTAs are being advertised for around the £2,000-£3,000 mark for a full time course including exam fees. Of course, if you do a full time course it is quite likely that you'll have to travel somewhere or live in another country so you can add the cost of flights, accommodation etc to that. I did my DELTA by distance with BELL and I think it was around the £3,500 mark. If you're lucky, your employer may be willing to pay for some/all of the fees.
In 2006 Masters programs cost about the same. They have subsequently increased in price and I've heard they will shoot up in the near future to match BA courses, -though this could be a rumour. I luckily did my masters in 2006 for about £3,600. The same course now costs over £4,600. On top of this you have the loss of earnings for one year, the accommodation and living costs. This makes the number of people able to even think about doing an MA much smaller, I imagine. The part time option for the same course comes in at about £7,000 over two and a half years. There are, though, scholarships available it seems.
There is a good chance that if you choose a fairly big university and have decent qualifications, there may be chances to work in the English Language centre on campus. There are at least 4 people where I currently work who were in that position.
Order of acquisition
There are a number of MA TESOL courses which offer exceptions for DELTA holders. That is, if you have a DELTA you can receive credit for a portion of the MA without having to do it. The list ranges from nothing (Unis not on the list) all the way up to 60 credits. Rather ironically, Cambridge, producers of the DELTA, offers nothing. It thus makes sense to do the DELTA first IF you are planning on going to one of these universities. I did the DELTA second and it worked out for me because, by that time I had a full time job in the UK and the institution paid some of the fees. I did try to start the DELTA abroad but for module 2 you will need a trainer and it's pretty tough trying to find one in Asia.
Although it's sensible to do the DELTA first, it might be easier (as it was in my case) to take a year off work when you are younger. If you get a DELTA and then get a well paid job you might be more reluctant to leave it to start an MA with no promise of there being a job at the end of it. However, the longer you wait to do the MA the more you'll probably get out of it. That is, you'll probably have a better idea of teaching and more experience to give you a better idea of what it is you want to focus on.
It's also perhaps worth adding that as module 1 and 3 are exams, you can enter by yourself without actually doing a DELTA course. So, you don't have to take the course to apply for the exams and if you feel confident you might find this is a good way to save money. I will add that the exam has some very odd expectations in terms of answers, so make sure you aware of these if you plan on doing this.
What you will learn
The DELTA course is 3 modules. The first is an exam in which you will have to define terms like "notional functional" and "unbounded morphemes" and be able to say who started the "silent method" and what it involves. Why this is important for a teacher to be able to do is anyone's guess. The test, which is actually two 90 minutes exams, does have a few useful sections. The section in which you have to analyse and correct a student's work seems pretty authentic to me. Also the section in which you must analyse a test and find its faults is quite useful...though you inevitably start to think about the flaws of the DELTA exam itself.
The second module is the practical part and this is the real meat of the DELTA. you are assessed over two months and have to produce a huge amount of paper. There are five lessons (including the experimental) four of which are observed and one of which is observed by a n external candidate. If you fail that then you fail the whole thing. you do have a chance to retake this though as I did. One complaint about this module is that it doesn't explicitly tell you what good teaching is, rather it just seems to allow anything so long as you can justify why you did it. Another problem is the huge amounts of writing you have to do. 5x 2,500 essays plus a detailed lesson plan each time 500 word post class reflection and a 800 word linking piece between the essay and and the lesson plan. There is also a personal development essay of about 5000 words, which you cannot fail and which is full of the kind of meaningless pseudo-babble that I personally despise. "I feel I have developed as a teacher and met my objectives" --ugh! (edit: I might be being a little harsh here)
The third module is quite interesting. It is a long essay which is divided into sections and staged quite cleverly so that if you mess up the start you're pretty much done for. You have to firstly do a needs analysis with a class. Using the needs analysis you devise an exam for the students to test their abilities and then finally you create a syllabus/15 lesson course around your findings. It's quite a neat intellectual challenge though I did have some issues with it as well. The literature on needs analysis is a bit fluffy and lacking any real scientific basis. It just seems likes opinions dressed up with academic language. It also seems a bit questionable to me to take time out of lessons to test students for a course that, in many cases, they will not actually ever do. I wonder how ethical this is?
The DELTA has also recently introduced a 3rd module for managers and those wanting to be a DOS which seems like an interesting move.
Though the DELTA curriculum is standardised, MA courses are much less so. It is also worth remembering that two holders of an MA TESOL could have studied completely different things. For example:
[course A] Methodology/ Second language acquisition/ Intercultural studies/ sociolinguistics /phonology/
[course B] Syllabus design/ testing/ psycholinguistics/ corpus studies/ grammar
Therefore it's probably worth thinking about what you want to study and trying to find a Uni which offers something along those lines.
in short, people can and often do fail or give up the DELTA. It is very time consuming and I wasn't always convinced I was doing anything other than busy work. It would take some spectacular skill to manage to fail a master's degree. Universities are not very good at failing people and short of not submitting work or plagiarising it's a good bet that you will pass.
It's a bit of a risk doing either one or the other because there are some jobs which prefer the DELTA and others, the MA. There isn't really one choice that will satisfy everyone and as the job market gets more competitive, the number of places asking for, and getting candidates with both is increasing. The place I work used to require a DELTA or equivalent qualification. Now they state DELTA essential despite it being essentially an academic department.
Generally speaking the DELTA will get you further. The DELTA is the British council's baby and hence they will look favourably on people with it. The DELTA is also more respected as a 'practical qualification'. Jobs in EFL in Europe will more often require the DELTA than an Master's. If your goal is university work in Asia, (particularly Japan where the British council doesn't have a great presence) the DELTA is quite often unheard of. A search of Gaijin pot (Japan) brought up about 3 jobs which asked for a DELTA (an then they were just listed as 'desirable') and 1 on the TEALIT (Taiwan) site. A search for Master's degree's brought up slightly more but this time they were listed as essential.
It is worth noting that a master's degree is not the guarantee of lucrative university work in Asia that it once was. Almost always the departments will want people with a MA TESOL or applied linguistics and almost always they will require some published papers. Taiwan is also quite fussy about what kind of master's degree you got and they will want it to be officially stamped by your university notary and then by their 'embassy' in whichever country you are from. they will also not accept MAs that were done part-time or those which are over 3 years old. This legislation is apparently an attempt to avoid fake degree certificates.
The DELTA gives you a chance to examine your teaching. Unfortunately as there is so little actual solid theory in EFL teaching you can't be convinced that what you're being sold is actually worth anything. OK, so now I know what a notional functional syllabus is, but I'm not sure if I should be teaching one or not. The module three essay at least gives you the ability to try to set up a course doing a needs analysis and designing a course around the results. It might not be great but it's perhaps the best we've got a this moment. For those with an MA though, the theory side of the DELTA might seem a bit superficial. Getting a DELTA though has some kind of magic aura associated with it. For English teacher's it's like being a war veteran or a karate black-belt. You just exude confidence and authority (whether or not you have any is another question...)
I personally preferred the MA, so I'm probably quite biased but the MA allows you to investigate whatever it is you want to investigate. The DELTA essays do allow this as well, to some extent. In short the DELTA seems to say "this is how is it" whereas the MA says "why is it like this?" I felt I got a lot more out of the MA and though I can't say I became a better teacher by doing it (after all there is no practical element on most courses) it (cheesy cliche) enhanced my world view.
Any questions or correction please comment. I would love to make this article more general as at the moment I can only go on my own experience.
Here are a few threads discussing the topic in more detail.