The answer pretty much depends on your choice of branch. Last year i had the same dilema (AIR 1789) and i opted for IIT patna CSE and it has really been worth it. I think with the new well built campus and good faculties , IIT patna is slightly better than IIT ISM dhanbad specially when it comes to branches like CSE and electrical.
If you are interested in mining then without doubt go for IIT ISM Dhanbad.
Overall i think you should focus on the status of the branch you want to pursue and check it’s placements and faculty profiles in order to get a better idea about the situation. Moreover talk to seniors of those respective colleges and ask about the curriculum to get more insight.
More or less both institutes are almost equal and you won’t regret joining any of them.
Good luck !