I own both
I bought my girlfriend a pink camo Walther P22 which is fun to shoot. I paid just under $400 for it. All the talk about it being ammo sensitive are accurate. I cannot count all the FTF's & FTE's I have had with it. Also more than one round has been lodged in the chamber. This is a real pain in the you know what. The only ammo I let her put in it now is CCI Stingers, because every other type of ammo I have used causes some type of failure/ mag.
I wanted a semi-auto .22 pistol for myself, but wanted something a little more accurate. I ran across an older 22/45 made in the 1990's that had a target barrel for $250. This handgun had been well taken care of and appeared new, so I bought it instead of a new one. I have never had a failure of any type with this weapon, and it is much more accurate. Also if you want to upgrade the barrel/ action and make it a tree rat suppressor Tactical Solutions sells some nice upgrades. You can also mount a scope on a 22/45 because it has a bolt, not a slide.
I would say that the 22/45 is hands down the better option for accuracy, usability, and durability. The P22 is lighter, smaller, and still fun but...... FOR THE BETTER WEAPON AT A CHEAPER PRICE (AND MORE OPTIONS), GO FOR THE RUGER 22/45.