I have had a number of MK III 22/45's and have bagged more rabbits with them than I can remember. However, I would suggest the standard MK series, not the 22/45. Unless you can find someone manufacturing a metal lower for that gun, I don't think that it would be a wise long term survival gun. Working in local shops I have seen way too many 22/45's come in with a bowed or stressed magwell from someone putting the gun back together improperly. If it is in your budget, see about getting an aftermarket barrel for the Ruger, you can find great quality, accuracy and a much lighter weight by doing this.
Also, for long term survival, the Ruger makes for a great candidate due to it's simple, inexpensive and (if you are familiar with the gun) easy to replace parts.