There aren't too many Airsoft brands on the market that make MP5s and fewer still which devote themselves to an entire series of MP5s the way that Cyma has.
Cyma's 120 rds (also known as "Mid-caps") is the perfect blend of quantity and function. The 120 rds mag doesn't rattle or need to be wound like a 240 rds mag (hi-cap) does. I've tried all 3 tiers of MP5 magazine and can easily say I'd rather take a vest full of mid-caps into a skirmish than worry about winding a hi-cap during a firefight; especially as the MP5-k series is designed as a Close Quarters Battle airsoft gun, the winding or rattling of the BBs in the hi-cap mag will easily give your position away.
If MP5s are your weapon of choice, buy a half dozen of these and don't look back!