Hello himanij1495, see below, I am an engineer with 9 years experience and also trying to decide between an MEM and an MBA:
1) some schools do accept a GMAT, but most MBA programs now accept GRE also, so find the program you want and see what is preferred there, also if you can take each test without studying and see where you score higher, I would advice you to study for the one you do best at without studying.
2)simple answer is it depends, an MBA is a known degree, MEM is still new and is not accepted out there as much, if the schools your applying to is ranked top 50 I might go for an MBA, otherwise choose an MEM at a top school, and do your research.
3)MBA from top school you can expect to make 100+ with a couple of years of experience, MEM depends on the industry, but with two years experience from a top school you can make up to 80K.
good luck