I would say that pursuing Material Science from IIT Gandhinagar will be a better choice as compared to pursuing Metallurgical Engineering from IIT BHU.
You need to compare various aspects of both the institutes to reach a fair decision. The parameters that you should consider for comparing are as follows.
If you see the course structure and specialization, BHU is the oldest center of study for Metallurgical Engineering hence has loads of data available. But on other hand, less data is available for Material Science at IIT Gandhinagar as it is a growing discipline in the Meta field.
Academics: (Winning Institute - IIT BHU)
Here is the list of subjects for Metallurgy at IIT BHU with the first 5 being the core subjects:
Facilities:
Now coming to the facilities, IIT BHU offers you a long list of facilities for this particular program which are:
Faculty:
The faculty at both the institutes are quite good, to know more about the faculty you can visit their official websites:
Placements:
Now, coming to the placements, both the institutes offer moderate placements in these fields. Most of the students switch to Ph.D. from India as well as in international institutes.
All in all, it depends on your interest and goals. So, make such decisions carefully.