Well, you already know about the inability to swap grips on the 22/45, and the different grip angle. If you want to be able to mod the pistol for Bullseye shooting, I think a MK III or used MK II is a no-brainer.
I got the 22/45 MK III myself, in a 4" heavy barrel with adjustable sights. I didn't want it for a target pistol and like the 1911-style grip angle. The polymer frame IS thinner to hold on to, which is a bonus for me as my hands are on the small side.
Accuracy is great. Once the weather turns nice this spring, I'll give it a workout with something better than Federal Bulk Pack, but even with $9/550 ammo, it will shoot great, at least to 50 feet.
And don't sweat the assembly process. It's not rocket science. Once you learn the little trick of where the strut goes, it pops right together, first time, every time. The only disassembly difference between a MK II and a MK III is that you need a mag to squeeze the trigger to dry-fire it.