I will do PU this year so how to pursue MBA if i want to take science or how to do MBA through Commerce.what are merits & demerits of both?

is it better to pursue MBA through Commerce field or Through Science Field?? Is it safe to do B.E and then an MBA because i can keep B.E. as a fall-back if I dont get my preferred choice of MBA university or course? or will it be difficult to tackle CAT exam after studying Science?