i found the MK1 ever so slightly tighter . more important will be head diameter of said pellets. also helps tighten groups. in the end , you will have to experiment just like the rest of us. but that gun shoots most JSB quite well.
My vote is for the MK 1. I noticed that the skirts on the MK 1s are thinner and help obturate easier under air pressure (my theory, anyway). I've found the their accuracy is slightly better than the MK 2s.