Hi

If you are sure about going for space research, IIST is the obvious choice. You will get direct exposure to facilities like ISRO in your studies there. However, if you are not very clear about your field of specialisation, and want to go for research, I suggest you should pick IISc. It can provide a more multi-faceted experience to a student, and can help you narrow down to the field of your interest, in which you can later pursue higher education.

Hope this helps!