Firstly congratulations for getting such good result.You have really scored very well. It shows your hard work and determination.

Now coming to your question which one should you prefer IIT or NIT? Well it depends solely on your personal interest ,if you are interested in computer science branch and you're getting a top NIT ,which you will definitely get on your rank,then I would suggest rather than going into an IIT with branch in which you are not interested will not be a good decision. considering your rank you can easily get NIT Trichy ,NIT surathkal N,Warangal even NIT Calicut which are among the top IITs.

If you are getting the branch of your  interest in a very good IIT, then definitely should choose it but if you have to compromise your choice then go with an NIT( southern NITs) .It depends finally on your interest and the college which you are getting.

I hope you will do great in your life.

All the best.