[Shosheski]

I personally use the M4A4, but depending on your rank and skill its different.
The main difference between the A4 and the A1-S is the ammo capacity, and the accuracy.
The M4 has 30 ammo in one clip and 90 extra. It makes a lot more noise and its better for higher ranked players because of spraying. Its a little bit less accurate than the A1-S. If you are playing rifler or entry fragger you are better off with the M4.
The A1-S has 20 ammo in one clip and 40 extra. Makes a lot less noise and you will not be seen on the radar as much. It's more accurate than the M4. If you are playing rotator or stalker(lurker) the A1-S should be better. 
In other words for lower ranks use the A1-S.