Better = They are both accurate and dependable. I like the 22/45 because of the grip angle and mag release. Others prefer the steeper grip angle of the MKII. (Couldn't understand why anyone would prefer the mag release of the MKII though). I have some of each, and know others who do as well. One plus for the 22/45 is the little takedown lever is easier to flip out for some reason.
Accessories = Grips are a big challenge if you don't like the factory 22/45 grip. Other than that, you can sink as much into the one as the other.
SoLow mounts = I have only bought them directly from the SoLow website................chim