CLAT and Btech are 2 different streams. In case you are interested in law studies then after 12th you can pursue 5 year integrated course in law that is BALLB but if you are interested in Btech then there are different exams to prepare and appear for.

Though there are integrated course which have Btech and LLB together. Institutes like UPES, Sharda University provide this course. So if you have interest in this then you can further pursue it. 

Based on your interest you can decide which career you have to choose because the comparision does not fits between these 2 courses.

Through CLAT you can get into top national law schools in India and in btech through top engineering exams you can get into top NIT, IIT's etc.

Scope in both the fields is good and vast as it depends on your interest.

Good Luck.