my 2cents for what its worth, is based on first hand from 2 friends that own a ruger from one and a walther from the other, no experience with the sig.
The walther was used, the ruger was not, so some of this might be that the walther was used. And i personally got a S&W 22a, but it has a very large grip that might not feel good to your wife, you can get it with a 7" barrel however and with that and a cheap red dot from walmart i can hit a 1.5" swinger at 50 yards 8 out of 10 shots, easy, with no magnification on the red dot, and it also has a full rail for mounting different optics so you can even put a scope on it if you wanted. Down side is, with the 7" barrel its huge, bigger then my pt1911, so you wont be carrying this thing easily, you can get it with a 4 or a 5.5" barrel however, and swapping them takes all of 5 seconds. So now that the shameless plug for my fav 22 pistol is out of the way, on to the reviews of the ones you do care about.
The ruger shoots very well, and next to mine or a ruger mk3, i would say its right there with them, great shooting gun, doesn't have the aftermarket customization that the mk3 does, but then again, what does really. not sure about availability of different barrels and such as the friend that owns it is perfectly happy with it out of the box, at 50 yards with iron sights he is about 50/50 on hitting the same spinner i mentioned above.
The friend with the walther however, hates it, it is the 6" barrel model, the barrel does look to be in good shape, but he cant hit the broad side of a barn with it, when we have shot it as well, the other friend and i agree that it shoots high right, so once aim was adjusted we could at least hit coke cans and closer targets with it, with some level of regularity but even with a pistol vise that my dad has we could not find a sweet spot that would allow it to hit that 50 range spinner at all, tried to hit it with minor adjustments for 100 rounds, and even when we did get it to hit the target, it would not repeat that 2 times in a row ever. the shots were going into the dirt behind the target left, right, top, bottom, nothing consistent at all. But again this was the pre owned of the 3 of ours, and that might have something to do with it. On a personal note, i really did not like how small it was, in my hand 2 fingers were always off the grip.
one other thing to consider is, magazine cost, not sure what the current prices on all of them are, but the S&W were around $18 each when i got mine, the ruger was around $20 and the walther was around $30 according to my friends.
and we all ended up spending about the same amount of money on them, the walther guy was just impatient and got the first one he liked the looks of without doing any research, and then taking a favorite practice of mine, finding a range that rents the one your interested in so you can shoot it before you buy it. always like to do that when possible even if i have to drive an hour or more to get to it.