JG M4 S-System vs. G&G UMG?

I am buying a airsoft gun soon and am switching from paintball. Which gun would you think is better for a kid like me? I am 12 years old and about 4'11". My favorite gun is the UMP 45 but its more expensive than the JG. I also like it because it's smaller and lighter but the M4 got better reviews. Please help me!!!!