What should I choose : Preparatory course in IIT or B.Tech course in any NIT/IIIT/GFTI ?

Dera Friend, if you are getting an opportunity to get a seat in an NIT/IIIT/GFTI then do not hesitate and just go for it but if you are bent upon taking admission in an IIT only then you may have to drop a year, join preparatory classes and even then there may be no certainty that you may get your desired branch or not. You may also consider joining any one of the highly reputed institutions – SRM university, Chennai,........................VIT, Vellore,......................Lovely Professional University, Phagwara