It is rightly said that commerce is the stream people should take if you are interested in doing MBA as your PG. But there are few merits of opting engineering (science) as your field too and they are:-

1. Engineers are good in QUANTS section of CAT(which is entrance exam for MBA).
2. Toppers in CAT are engineers(Seventeen 100 percentilers in CAT'15 and all are engineers).
3. After four years of struggle in engineering it becomes comparatively easy to CRACK entrance exams of various PG courses compared to guys of other fields.

But there are few demerits too

1. Usually engineers suck in verbal ability which can be improved if you keep on reading a novel throughout your engineering.
2. You will put 4 years in engineering but a commerce guy will graduate in 3 years.
3. People are not able to handle this 4 years of struggle so decide wisely.

I know you didn't expect this answer but i am writing this answer as I have seen commerce people struggling in QUANTS( Obviously not all but many) and want you to choose wisely your future.

If you select COMMERCE as your stream then while doing your studies start reading novels, solve Quants so that you rise above an average commerce student going for MBA.

For CRACKING entrance exams, field that will help you is SCIENCE.

For surviving in 2 years of MBA preferred field is COMMERCE.

Choose wisely and All the best.