Is IIT Patna CSE is better or mathematics and computing at IIT Guwahati?

Whether IIT Patna CSE is better or MnC at IIT Guwahati, depends on what you want to do after your graduation. 

If you have an interest in pure and abstract Math and want to do research in Maths, theoretical computer science, Set theory, Group Theory, and Complexity Theory, then the IIT Guwahati MnC program is the best for you.

However, if you have an interest in coding and want to become a competitive programmer, or a software developer, and research in areas other than Theoretical Computer Science, then IIT Patna CSE is the right choice for you. Extensive research on every CS field is done here.

Basically, the academic workload of the MnC program at IIT Guwahati is too much with a very rigid curriculum that will spare you no time to focus on coding. No doubt, the MnC program too has excellent job opportunities in finance, banking, and software jobs. But, the demand for CS students is much more than MnC students and is expected to be increasingly so over the next few years. 

Common courses of CS and MNC include theoretical computer science, some being computer networks, and operating systems while the rest of the courses are related to abstract Maths/pure Math and are highly rigorous in nature. Contrary to popular belief, there aren't many common concepts between MnC and CS. 

IIT Guwahati will give you a slightly better campus life in comparison to IIT Patna considering its scenic campus and high-level fests organized by the institute.