Which is better IIIT Guwahati or CMI?

CMI and IIITG are distinct institutions, so, it will not be fair to compare.

CMI CSE course is theory-oriented whereas, IIITs offers a more practical-based CSE course. CMI is one of India's premier theoretical CSE institutes. The old IITs or IIITs are ideal for engineering or practical applications. Therefore, IITG is not the best for future scope at the large, but this has nothing to do with the institutions as it is a new institute and will take some time to establish itself.

If you want the engineering approach of CSE, IIITG will be better for you. The campus life as IIITG will also be better than CMI. Although CMI is ahead of IIITG in terms of competitive coding. Here is a brief outline of the CSE degree offered by both institutes.

If you are unsure, choose CMI over IIITG.