Ads long as you are talking about the Single Six I would suggest getting the one with the 22LR and 22 Mag cylinders. If I was to take it with me in it's "Survival Option" mode I would go with the magnum cylinder.
As far as supply of ammo goes, run down to Wal-Mart today and buy a couple/half dozen boxes of ammo and you should be good to go.
Any one who waits for the day after the shtf day to look for ammo is probably going to be out of luck anyway.
Stock up now for whatever caliber you plan to depend on in the future.