Hello Priyom,

The choice between the two wholly depends on your interest in the specific field. If you are interested in space stuffs then IIST will be far better than any other college as it will give you a lot of exposure to ISRO but if you are interested in space and galaxies and also you are a bit of technology oriented person then you must go for IIT as it will give you a lot of exposure in technical field.

Hope it helps.

Good Luck!