I'd go with the MkIII.
Reloading a 6-shot SA revolver is, IMHO, a royal PITA, so regular reloading of a 10-shot SA revo is out of the question for me.
OTOH, if the choice were between a MkIII and a 10-shot DA/SA revolver (e.g. S&W 617), I'd go with the DA/SA revolver. Besides, I've found my 10-shot 617 (and my MkIII for that matter) to be more accurate than any Ruger rimfire SA I've tried.