You should compare these two institutes based on the performance of the branch that you are interested in.
ISM is different from other IITs, in the sense that it is well-recognized for Mining, Geology, Petroleum, and other related branches. Other core branches like CSE, Electronics, etc. started after it got the IIT status. So, the institute will need some time to catch up with IIT BHU for these branches.
While IIT BHU has had many years to establish a brand name for these branches. It also exhibited many high packages over the years. Although the infrastructure at the institute is poor.
So, if you are interested in branches like Mining, Petroleum, etc. then you should choose ISM. For mainstream branches, IIT BHU is better.
Both IIT(ISM) Dhanbad and IIT BHU are premier colleges of India and both have been upgraded to IITs in the recent past. IIT(ISM) Dhanbad is considered one of the best institutes for earth science branches like Mining, Petroleum, and Geology. For these branches, IIT Dhanbad is unparalleled with any institute in India. But for the mainstream branches like Computer Science, Electronics, and Electrical engineering, the institute is currently in its developing phase. The infrastructure of IIT Dhanbad is far better than that of IIT BHU. But when it comes to placements and average packages, IIT BHU is a little ahead and it is ranked amongst the top 5 colleges in India in terms of placements with its highest package being of Rs 2.27 crores in the year 2016. The legacy of IIT BHU started as IT BHU and the institute is 100 years old so it has a vast alumni network.
The internet connectivity and wifi speed at IIT BHU is one of the best amongst any other IITs, so IIT BHU has an upper hand at this also. Professors are good at both the institutes but both not as good for research when compared to other IITs and have a strict 75% attendance rule. The nightlife of IIT BHU is far better than IIT Dhanbad’s as there are no restrictions at the hostels and the added advantage of foreign tourists increases the cultural diversity of the place. Both the institutes are very good and have a legacy of their own. To have a proper assessment of these two IITs, you should compare them based on the performance of a particular branch.