what should I choose BS mathematics and computing in IIT Kanpur or B tech mathematics and computing in IIT Hyderabad ?
Maths and Computing is different in different IITs.
IIT Kanpur Maths and Computing is different from the others because it has 80% Maths only, and very less CS.
IIT Hyderabad, has a lot of CSE as compared to Maths in their curriculum. They have the best syllabus to go for.
So, you should choose one according to your interest and inclination towards the curriculum and syllabus.
I hope this helps.