I agree with everything said about the Rugers. However I'd have to disagree with the comments about the Walther. I have heard of one or two stories about the slide cracking but have NEVER actually seen one or know anyone that has seen one cracked.
I have personally never seen one crack either, but my .22 pistols see alot of use. Where it might take me over a year to run 1000 rounds of 9mm through one of my guns, ill run 1000 rounds of .22LR through a pistol in 2 weeks easy, and have done it in one day. For this reason I prefer steel for my .22's, I really like the Ruger MK series pistols, I have a 22/45 and plan to get a MKIII hunter at some point.