This choice majorly depends on the student and his preferences. 

Seth GS:

• If you prefer studying a lot probably most of the time then opt for Seth GS. 
• They have a highly qualified staff and caters to KEM Hospital. 
• You will be studying amongst some of the brightest minds. 
• The hostel and accomodation facilities are the only problem, due to its location.

GMC:

• If you want to enjoy your college life, Grant Medical College (GMC) will be good for you. 
• It has the largest campus in Maharashtra. 
• GMC conducts the best cultural fests and events, be it sports, art or academics. This provides great exposure. 
• The staff here is also highly qualified just like KEM. 
• Students here tend to lean towards the extracurricular activities more than the studies. 
• The campus has 2 boys hostels and 1 girls hostel enough to provide lodging for all the students who apply. 
• The hostel provides facilities like hot water, pure drinking water, beds, tables, etc. 

Most people will claim that KEM is better for your career. But at the end of the day you also need some extracurricular activities to engage in and avoid the monotony.