Both CSE at MNNIT and Mechanics at Mechanics IIT Mandi is highly esteemed programs. The choice of the institute depends on your personal interests and career prospects. The branch you choose is more crucial than the institute from which you pursue it. So make sure your passion is the driving factor in choosing your branch. 

If you have an interest in either CS or Mechanics go for them respectively. When you pursue a stream of your choice, the chances of you becoming successful increase as you are motivated for a long time. But if you are not certain about your inclination, then my suggestion is CSE at MNNIT. CS program is well-regarded and is one of the highest-placed branches in MNNIT. 

Personally, I would never recommend anyone to go for IIT Mandi as it is a relatively newer college than MNNIT because of which it lacks in terms of alumni and industrial ties. Additionally, the faculty of IIT Mandi is not at par with old colleges. The only reason one can join IIT Mandi is when they are adamant about the IIT tag. Else, MNIT CS will not disappoint you.

Ultimately, it is important to research both programs and consider factors such as location, campus culture, course offerings, and job prospects before making a decision.