Both IIEST and NIT Sikkim are excellent institutes for engineering in India. There are several aspects in which one of the two institutes is ahead of the other. The points below elaborate on those factors for both the institutes to give a brief overview of both the campuses.
Overall, both the colleges are decent in their own aspects, providing several opportunities to students in fields of placement and research. It is on you to do good research about both, observe the key highlights, and then make a decision.