Usually most students prefer M.Tech. to M.S. in the same University. Reason is mainly the fixed duration of M.Tech. compared to not so fixed duration for M.S.
- M.Tech. is usually 2 years duration and rarely it gets extended.
- M.S. usually takes 3 years, but really good students finish in 2 years.
- M.Tech. students usually need to take about 10 courses while M.S. students need to take about 3 or 4.
- M.S. students need to publish a research paper while this is not so strict for M.Tech. students.
- Many of the M.Tech. courses are fixed and a student is forced to take courses from different areas while for M.S. all courses can be only from his area of interest.
- There is almost no difference for placements for M.S or M.Tech., the only problem being the course ending time. Many M.S. students prefer to attend placements in 3rd year to be safe.
- Good students doing M.S can get better recommendation from Prof for Ph.D. or research jobs.
In short, if you are not sure prefer M.Tech. If you are really into a research area prefer M.S. Opportunity wise not much difference. I would always prefer an M.S. from an IIT/IISc compared to M.Tech. in top NITs even if its risking 1 extra year. Many people do the other way. Environment is so different between the two. NITs are just a better state level govt. college but top NITs do have placements comparable to IITs. Still study environment differs a lot and in IITs one does more useful stuffs in 2 years which makes a better professional.
Research is different from course oriented M.Tech. In most places we have M.Tech. (TA) where TA is for Teaching Assistant-ship and you are paid for helping conduct bachelors courses (like evaluating papers) or managing the labs. In M.Tech. (RA) you are supposed to aid in some research and this might have to do with running some experiments, coding something etc. Pure Masters research would be like a starting to Ph.D. and you will be just focusing on a select subject and finding and solving a problem which has importance in the area.