It totally depends on which IIIT or NIT the student is applying for. If we talk about placements, faculties and the infrastructure, then some of the IIITs are better than NIT. It also depends on what a student wants from the course. IIITs are preferred over NIT by most of the students  who join IIITs have better rank, hence collectively they are better than those in NITs. However, like everything else there will be exceptions.