Both are good choices and more or less the same gun. The major difference is that the grip angle of the Mark III is more like a Luger where the grip angle of the 22/45 is more like a 1911. Beyond that, the only major difference I can think of is that the 22/45 has a polymer frame vs the Mark III which is steel.
Depending on what model of 22/45 you get, you probably will NOT be able to remove and change around the grips. I know Ruger produced a model of 22/45 where you can swap the grips, but I don't know if it's a standard thing now on the new pistols or not - regardless, there is *lots* of old-stock 22/45 that you won't be able to change the grip on, so be sure to check and think about it. The Mark III's all have changeable grips.
For what it's worth, I'd go with the stainless steel. I'm starting to like that finish more and more, plus the plastic hard-case Ruger ships with seems to wear the bluing off of the regular finish.