Hey Prashant,

Speaking of both the institues, both are good in their own standards. But since you have asked the difference I'll compare it point by point. It is my analysis so I will try to represent each institute in their own terms, good or bad.

1) Both the universities give admissions to the students on the basis of their JEE Mains score. 

2) The average placement in Thapar is better than LNMIIT but the students getting placed in top positions is almost the same. Also another point is that the placements at LNMIIT are improving each year. This year highest pay package in LNMIIT was of INR 25lakh p.a in Amazon. But almost all good companies like Microsoft, Amazon recruit from both these institutes irrespective.

3) If you are into CSE than the coding culture in LNMIIT is better for your growth. This can be seen from the student participation in the monthly contests. Another fact is that LNMIIT produces GSOCers every year, which is a plus point. 

4) Speaking of the faculty, LNMIIT has a slightly better faculty than Thapar and almost every faculty member in LMNIIT has a PhD degree. 

5) About research facilities, well this is a thing where LNMIIT is better than many other private institutes. Also in LNMIIT many of their students are involved in research and get their papers published which surely helps them if they are interested in higher studies abroad. 

6) Last point, co curricular activities is good in both these institues.

Good luck