Hi,

See, both are premier institutes but I would suggest IIT Bombay, which is the most popular choice among students for M.Tech in CSE. Research facilities and academics and faculties are good in all the old IITs, so are placements. But do remember, Mumbai is the 'financial capital' of India, hence a number of companies flock there. The average package is around 12 lakhs and you can get as high as 25 lakhs. Also, if you are interested in doing a Ph.D. in Computer Science, expect very good recommendations. IIT Bombay has consistently been ranked high in various international rankings. 

The IIT-B culture is surely incomparable. The picturesque campus is in Powai with lakes and hills. Techfest, IITB is the largest science and technology festival in Asia. Also, Mood Indigo is Asia's largest cultural festival. Studying at IIT Bombay is surely a matter of pride!

Hope this helps.

Cheers!