i had the mp9 got it for xmas, loved the thing alot till it had to be left at customs a few days later, its a wonderful gun but i think for what it is it shoots too hot, and it jams easy you have to keep this thing as clean as an ER i would recommend the m4 i dont have and experience with them but as far as versatility goes i believe the m4 would be worth it. the mp9 is really only a secondary