Among the colleges present in the lot, I would advise you to choose NIT Silchar over NIT Agartala. I have a few friends at the institute and I have heard good things about the institute.

Here are a few of the reasons why NIT Silchar is a better alternative. 

1. IIIT Kota and IIIT Kalyani are pretty new and hence these are generally not the first choice.
2. NIT Silchar provides some extraordinary facilities like a supercomputing center and exchange programs with the University of Illinois.
3. For those who are interested in Computer networks, parallel algorithms, etc, and want to pursue their career in these fields, supercomputing provides a very good foundation to get into these fields during the undergraduate coursework.

The above reasons should be enough to choose NIT Silchar.