Uh, Its really up to you on that, there is no 'better' in airsoft. It all just ends up going down to what you prefer over the other.
If you wanna mod the crap out of your gun with flashlights and the such, go with the m4. If you like the badass look, go with an AK47 or whatever. Of course people can go down to the differences between the v2 and v3 gearboxes...
Its really just up to you