There is no better college of the two because both are equally good in terms of placements and faculty. Both have their pros and cons. But here I will just cite some pros of both colleges that might help you make a better decision. 

Pros of GIM

• Offers a comprehensive learning experience 
• Many student-operated clubs and committees 
• Fun-filled campus life 
• Top-notch infrastructure
• Better placement opportunities and exposure to industry projects
• Smaller batch size therefore a better chance to grab good packages and lesser competition.

Pros of KJ Somaiya

• KJ Somaiya a location advantage for students from Mumbai, Maharashtra 
• better ROI

For more details you can talk to the alumni of the respective colleges.