Hey,
It depends on the courses  you want to pursue. Both be the top colleges of du and stand in somewhat in same respects but have different environment. Ip college be a north campus college where as gargi college is a south campus college of du.
Talking in terms of infrastructure and campus well ip has it all. Ip college has great infrastructure with beautiful campus around. You'll enjoy a peaceful life with less election stuffs in ip.  Coming to hostel facility ip has provided with two hostels both being amazing and less problem.  Gargi has not been provided with hostel facility.
Also the commutation, as ip is easily accessible having the  civil lines metro station near to it.
Rest it depends on you as what course you want to pursue. In both colleges the faculty stand somewhere at the same places .
Good luck!