though iit kanpur and iit delhi are famous for their computer science department and iit bombay for its comp science placements but still you should do a detailed research for your mtech.
By research i mean you have to visisit the websites of these institutes and carefully look after the following points::

    
the number of faculty members in the department.
    
experience and qualifications of the faculty members.
    
the projects that are run under the department
    
the no. of research papers published every year by the dept.
    
the facilities and laboratory modernization for research
    
the awards and recognition that the department and the students and faculty have gained so far.
    
is there any big project running under the dept. etc etc.

look into the above mentioned points and satsfy yourself that you are in best place.Placements are almost the same in all iits and placements.
all the above informations are clearly stated and available at the websites of the iits.