The 12x6 has more thrust by high rotation speed as the 12x3,8, but the 3,8 gives more pressure by lower rotation speed.
If you want a stable hover platform, the 3,8 or 4,5 is better, if it is a heavy copter, than the 12x6 is better. If you want a fast copter for acrobatics, the 12x6 is better...