Hi there!

Keep in mind that I’m answering this from IITKGP Mathematics and Computing (MnC) department’s perspective. it's getting to vary w.r.t other IITs but not an excessive amount of .

In terms of placement, speaking on general terms, CS placements any day will have an whip hand over Mathematics and Computing. Well, there are always exceptions.

“Let the CS guys get placed within the first three days of placement then , we’ll get our chance needless to say .”—A typical MnC guy.

Courses. most the CS courses are taught in MnC department but there’s a difference. Mnc courses are more inclined towards Mathematics and not much coding. it's you who will got to code. The professors in MnC department won’t pay much heed.

CS department will provide you with many program to code. Coding will become your cup of coffee.

Interests. You say you're interested by both subjects. Mathematics is all different after coming to IIT. rather than solving skills you'll need more of mugging up. I even have seen many of my friends lose their interest in Mathematics after joining. I’m not trying to intimidate you but Mathematics here is indeed different from that of your school days.

If you're interested by both subjects, more in CS, choose the CS department. you will always take additional courses in MnC department to satiate your desire for Maths.

Thanks.