The biggest difference between the 22/45 and the MKIII will be the weight (if compared between same length barrels between the models). The lower section of the 22/45 is plastic, the lower frame of the MKIII is steel. Based on specs I saw on the Ruger site, that's probably a 7-9 oz difference.
The 22/45 grip angle and "controls" supposedly mimics a 1911.
The slide lock (bolt lock in Ruger terms) on the 22/45 is the same style round knob that the safety has. On the MkIII, it's a lever, and a thin lever at that.
The grip on the 22/45 is thinner, and you better like what you get, the grips are part of the frame ala an XD.
Grips on a MkIII are easily changed, and the grip is wider.
Both models are available with the tapered barrel (fixed sights), the fluted bull barrel (Hunter models, fiber front sight, v notch read sight) and the Slab barrel (Target models, standard front sight, rectangular notch rear sight).
As to the heavier barrels (Hunter / Target) they will reduce muzze flip (I hear the .40 cal guys snickering). One touted advantage is the mass of the barrel will heat up less than the tapered barrels. So how fast and how much you going to shoot?
Best bet is to hold them and get a feel. Those that tout the 22/45 as a practice plaform for a 1911 ignore the facts that the trigger pull is different, and obviously the recoil. You could get a 22 upper for some models of 1911s, but they may end up costing as much as the Ruger.
Two things about the 22/45, it will be cheaper than a MkIII and the balance of it with a longer barrel will be different than the MkIII would be.
Alas, both the new models come with magazine disconnects and the loaded chamber indicator, some have argued that both contribute to feed problems. The magazine disconnect does add a couple of more steps in a field strip than a MkI or MkII had.
I've got a MKIII and was considering getting a 22/45. Magazines are different and when I release the magazine on a 22/45 in the gun shop, I had to position my hands in different way to prevent the magazine getting hung up on my finger - wouldn't drop out cleanly.
I'll probably end up getting another MKIII (different model than what I've got) because I've already got extra mags that will work and I am willing to spend the extra bucks for the feel of the steel. But if the balance and grip work better for you, the 22/45 will be the cheaper route.
Last edited by Old4Eyes; 02-02-2010 at 10:03 AM.