I have a few friends at both institutes so I can give you some idea about them. Both IIIT Guwahati and Thapar University are very good engineering institutes, but the former will be a better choice.
Thapar University is a reputed private institute with good infrastructure, an updated curriculum, and a well-developed alumni base. But when compared to IIIT Guwahati it falls behind.
If we check the recent placement statistics for these two institutes, there a big difference can be seen. The average CTC offered at Thapar for recent years has been around 7 LPA, whereas for IIIT Guwahati the average CTC is around 10.2 LPA.
The final decision is yours. Consider all these facts and choose wisely.