If your choice of course is information technology, then you should go for IIIT Allahabad. Both IIT Bhubaneswar and IIIT Allahabad are good colleges but the choice comes down to the course you are interested in. Joining a college with the hope that you will get your branch changed is risky. Do not make the decision to take IIT Bhubaneswar just for the IIT tag. In India, metallurgy has less scope as compared to IT. Also, IT from IIIT Allahabad is considered among the best in India.
There can be two possible outcomes if you go for IIT Bhubaneswar Metallurgy.
Outcome 1- You study hard in your 1st year to score 9+ CGPA and succeed in changing your branch to CSE (keep in mind that it is difficult to change branches in IIT). Then you get your IIT tag and choice of branch.
Outcome 2- If you fail to change your branch you will be stuck with Metallurgy which has less placement offer in India.
So, don't go for IIT Bhubaneswar Metallurgy just for the sake of IIT tag. As IIIT Allahabad IT is among the top IT programs in India with decent placement.