There is a branch change provision at IIT Mandi which is very lenient too. So if you are confused with your branch and you wish to change your branch in the future, then you should choose IIT Mandi.
One of my friends was in a similar situation a few years back and he opted for IIT Mandi for its flexible branch change policies. He was able to change his branch to CSE. To change your branch to CSE, you will need a CGPA of around 8.6.
IIT Mandi has a 50% branch change allowance whereas at IIT Roper it is 10%. To know more about the branch change rules you can visit the official website.