Hello,
Its good that you are getting NIT and IIT both. Preparatory courses will take one of your year from your life, and you will get only least preferred branches in IIT like marine, textile and branches. That branches dont have any good placement record as the other branches and also they dont have various scopes for future.
Chemical Engineerin is a good engineering stream. It has a wide scope. Also, NIT Surathkal is among top 5 NITs. Hence, you should go to Chemical Engineering at NIT Surathkal.
According to me NIT Surathkal is best option for your future career because campus,placements,teaching everything is best.

I hope this would help you. For further queries please feel free to write me down in the comment box.