See if you want to go for the ISRO then it would be better for you to go for IIST the reason behind it is because if you score above 8.5 in your UG course in IIST you get a chance to get directly placed in ISRO. Now NASA have a separate procedure and one have to apply corresponding to the vacancy created there. So if you have the passion to go direct in ISRO then go for IIST. Now if you wish to go for IIT then IIT KANPUR have the best infrastructure and the faculties for Aerospace Engineering. The sequence is followed by IIT KGP, IIT B etc. But this doesn't give you a certainty to get job in ISRO but you can go for ISRO based on the ICRB entrance.

To know more about IIST visit the link below:

https://www.careers360.com/university/indian-institute-of-space-science-and-technology-thiruvananthapuram

To know more about IIT kanpur visit the link below:

https://www.careers360.com/university/indian-institute-of-technology-kanpur

I hope this answer helps. All the very best for your future endeavors!