What should I prefer: mathematics and computing in IIT Kharagpur/Kanpur/Guwahati, maths in IISc, Bangalore or computer science dual in IIIT Hyderabad?

Among your choices, you should choose in a manner as briefed below - IISC>IIITH>IITK

When you are enrolled in any of these institutes, your hard work is considered to be more effective than your choice of institution. Your hard work as a whole is the only standalone concept in your career. It also depends on what your subject choice is. 

• If you are not good at mathematics, then choosing IIIT will be a better choice than the former. 
• Similarly, if you are not interested in computer science, then you should opt IISC. 
• However, if you share enthusiasm in both the subjects, only then you should consider yourself admissible to IIT.

I have many friends who showed initially showed interest in CSE due to its programming culture but later in their life, they end up choosing mathematics. It is because they discover their vivid inclination towards the subject. It can now vary from student to student and their further choices as a candidate in higher studies.

My sister similarly wanted to pursue mathematics later in her years and chose IIT Madras as her first preference. Many suggested her to opt for IIT Delhi as it has a great curriculum than any other IITs. However, the primary reason behind her choosing IIT Madras was due to the fact that she was more inclined towards Pure Mathematics. 

You, as a student, should not prefer a college over a course. Run a detailed analysis of each of the courses in different institutions. Only then, deduce from the results.