Whatever is standard issue is what you will receive unless you go on to specialised units or specialised training, and even then you'll have specialised standard issue.
Im pretty sure the Marines use M16a4's now... they are a bit heavier, but have a modified forarm grip and you can attach a scope in seconds.... thats about the only differences ive seen from the a2's
Most of the Army is not issued M-4's only the more deployable units. Most of the Army is isuued A-4's or A-2's. I really think that the M-4's should be set aside for the more specialized units such as Airborne and Air assault because of the compact and lighter design. The A-4 and the A-2 is more accurate and easier to shoot because lets face it just because you are in the military does not mean that you can shoot .
I have seen some people really not shoot all that well with the M-4 but they will light it up with the A-4 or A-2 because of the distance between the sight apertures the shot will be much moe accurate.