It totally depends on your personal interest, you have to decide it that, in which field you have more interest whether you wan to go for M.tech or or you want to go for the Management. If you give GATE exam you will do M.Tech. If you have interest in technical field then you must go for M.Tech. After doing M.Tech you can go for research of can apply for different jobs.

And if you have interest in Business field then you must go for MBA i.e you must give CAT and various other management exams. After that if you apply for job you will get a good handsome salary. Another opportunity for MBA students is that, if the students wants to go abroad then they can give the exam GRE.

The in depth conclusion is: Both these exams are highly competitive and toughest entrance exams for Indian colleges. But then, GATE lets you to qualify for best colleges in India which imparts technical knowledge in engineering domain, like IITs. Whereas, CAT lets you to qualify for the hallowed B schools of India like IIMs.