Hello dear

So basically there is no much difference between NISER and IISER , both are very good in all perspectives.

But as you  highlighted research in physics, you must opt the one which would help to give a great outcome at the end .

I'll tell the difference only keeping in mind - physics department

At IISER ,students will follow a common curriculum for the first 2years and then start to specialize ,whereas  at NISER students start to specialize in their second year .

The physics department at NISER is really very good and can expect very good outcome, if there is a  hardwork from your side.

But at the end it's your choice, both are good ,only when physics matters for researching NISER has good physics faculty compared to IISER .

Any queries drop down in the comment box

Good luck !