My opinion is that the MK II was the high water mark for the Ruger MK series pistol. It has the ten round magazine capacity, last shot bolt hold open, and heal mag release (I think that is a plus).
There is nothing wrong with the MK I, which I'd prefer over the MK III. The MK III has the unnecessary loaded chamber indicator (LCI), magazine disconnect, and internal lock. The good news is you can remove the LCI, and mag disconnect with a few, available parts, and it is not that hard to do.
If you can find a MK II for around the same price as a MK III get it, but they are getting a premium in good condition due to the above changes. If you can't find a MK II, get a MK III and do the changes if they bother you, if not, just shoot it.