It depends on your career goals. GATE offers more options for engineering students. It is an all-around examination. You can go for MTech, PSUs, MS, and FRM. If you want more options, GATE will be better for you.
If you want to do an MBA, after graduation, CAT will be the better option. Almost all top B-Schools accept CAT scores for MBA.
CAT and GATE being different from each other cannot be compared. As they both have their individual purposes. CAT is for MBA aspirants and GATE is for selection in post graduation levels of Engineering, Architecture, Technology.
Here are some facts about both of them:
CAT:
GATE:
It must be clear from the points that both CAT and GATE exams are best in their own way. Candidates willing to go for corporate jobs must choose CAT, while those looking for a technical career can opt for the GATE.