Since mechanical branch is famous in Roorkee but not in Guwahati, so I strongly suggest you to choose mechanical in IIT Roorkee and not in Guwahati. If your rank is coming under opening and closing ranks for cs in Guwahati then fill that.

I would not suggest iit mandi because of 2 reasons:

1) CS is not that good in those colleges and package is almost average to low rank nits.

2) Since it rank less than iit Guwahati.