UPSC NDA is most certainly tougher than CUSAT NDA. In terms of syllabus, CUSAT CAT would exclusively stick to Physics, Chemistry, and Mathematics. On the basis of these three subjects, your knowledge would be determined.
When we talk about UPSC NDA, your knowledge would be checked overall by observing your written exams which would be based on General Ability consisting of polity, geography, history, economics, English, and current affairs along with mathematics.
Whether it is via CUSAT or even NDA, both the exams would hand you the opportunity to work for the country. Do well!