A simple answer to this question would be IIT Hyderabad if you are looking for better academics and job prospects and IIT Kharagpur if you are on the lookout for a good social life.
Honestly, the course of math and computing at IIT KGP lacks the necessary requirements to impart true knowledge related to math and computing. The only reason why people still go for this course is the high placement packages offered at the end of five years. If you are adamant about joining IIT KGP, then I suggest you opt for a brand change and go for something that is more industry-relevant and worthy today.
The ultimate question is if you want to choose a college with good academics or good activities. Once you Are sure about the answer, Taking this decision will become much easier.