The debate regarding which is better NISER vs IISER is a never-ending one. It’s difficult to say which one is better with absolute certainty. Since there are many differentiating factors. You should choose your institute based on the field that you are interested in. Also, field doesn’t mean fundamental sciences like Physics, Chemistry, Mathematics, etc. All three of these basic sciences cover vast areas and there are many sub-fields. So, go through the sub-fields and figure out what interests you more check which institute offers better facilities and faculty base for that. That’s how you can choose the institute that will be better for you.