Hello student,

Preparatory course is mostly to lay the foundation of all the subjects and its concepts of JEE mains. However, if you have not solved quite a lot of questions during or after that course, you wont be able to crack mains and ultimately wont be eligible for admission in NITs.

If your main goal is only admission in NITs and not IITs, you can do your best in JEE mains, score great percentile there and can easily confirm admission and leave JEE advanced. The decision in completely based on you and your choices and preferences.

I hope this helps, All the Best!