Both are outstanding varmint hunting cartridges. Up until a couple years ago I had been using .223, specifically the Hornady 40gr V-Max bullet, with great success. I am now using .204 Ruger, specificaly the Sierra 32gr BlitzKing bullet. It is flatter shooting, and bucks the wind better than .223 in my experience. It is also more devastating than .223 for my purposes.
In terms of factory ammo, The Hunting Shack makes great ammo for both cartridges. Black Hills makes great .223 ammo, and for varmint hunting, I'd choose their 40gr or 50gr V-Max loads. Both are reasonably priced, about $25 or $30 for a box of 50 rounds.
I handload for both cartridges, though.