My Impact definitely likes the MKII's in my slug liner and the MKI's in the standard pellet liner that came with the gun. The difference between the two pellets isn't monumental but one does shoot better than the other. Shoot them both and use the one your gun likes the best.
MK2 should be a little bit shorter and sometimes a lot easier to load (depending on gun). Unless you are looking for competition level shooting, I doubt the MK1 vs MK2 will make much difference as far as accuracy goes, unless your gun yields skirt deformation or something.