Well, brother I understand you want to get into an IIT and you have scored which as per your category you can get an IIT.

But brother, going for a preparaotry course, I won't recommend to any of my fellow. 

Preparatory courses are very risky and not clearing at this, would result in expulsion from that college and this can really hamper your confidence and your year too.

So, I would recommend you to prefer NIT as per your JEE mains rank

Thank You