The choice of both branch and institute depends a lot on factors like your career prospects and areas of interest. However, since you haven't decided on a branch yet, I will assume you aren't yet sure of a branch and are open to exploring more branches. Don't worry, as most students clearing JEE Advanced face a similar situation. Personally, I will recommend you to go for IIT KGP over ISM Dhanbad because of 2 strong reasons
IIT KGP gives you the chance to explore and identify your area of interest. Even if you go for coding and later realize you don't like it, you can always go for non-core jobs that are in abundance at IIT KGP. But in the case of Dhanbad, you will have to stick to coding irrespective of your career inclinations. This is a major factor as it is your passion that determines your success rate.
IIT KGP has a more diverse group of individuals that are literal geniuses. Even if you are an above-average student you will face tough competition and thus will always strive to excel and grow. Dhanbad has an average-quality peer group and thus your growth might be restricted there.
I can only guide you to make a good decision, the final authority lies with you.