Help me decide which airsoft gun to get?

Out of the three, the M16 will be the most versatile. M4/M16 variants are the most common so just about everyone has magazines, batteries, other parts for them in case yours breaks or you just need a spare. It is also much easier to work on internally than the other two. I'd personally look into Echo 1 if I were you though. Much better track record, warranty and around the same price as well.