I have the mosquito and I really enjoy it. It did take me a while to find what bulk ammo it likes but once I found it I love the gun. Feels really good in my hands. When I was debating on either the p22 or the mosquito I read lots of bad things about the p22 so I stayed far away from them.
I would also look at the sr22, the ruger mkII or 22/45.
I would go with the Walther. I did have a Walther P-22, and it digested just about every brand of .22LR I feed it, but a close friend of mine has the Sig and says it will only fire CCI MiniMags properly.
While I love all the other Sig handguns I own, an early build Mosquito was not a great experience for me. Lots of FTEs and had to go back to the factory and still did not come back right. I eventually sold it, and from what I have read, the newer Mosquitos are much better.
Have a P22Q here. I have no complaints. It feels good in the hand, weight and contour. Eats everything so far. But it's just my opinon. I have no experience with a SR22 or GSG.
I have owned both the P22 and the mosquito. Sold the P22 to a friend, still have the skeeter. I might be one of the few people who will give a thumbs up for the skeeter. It is just a little bit bigger then the P22 which makes it useful for actually feeling like a real gun. Accuracy for both is similar. I actually shoot the skeeter better because it is a bit larger. Skeeters are kind of picky about ammuntion while the P22 was little roach that cycled everything I fed it.
Cheap plinker = P22
Cheap Training Tool = Mosquito
Please look into the Smith & Wesson M&P22. It is a great pistol and the same size as the M&P9 and M&P40, therefore making it a great cross-training pistol.
I have the Walther, the M&P and the Sig. I like the M&P first, the Sig second. However, I read manyk great reviews on the Ruger SR22.
I had a mosquito. Key word is had. I don't think I ran 100 rounds through it before I sold it. That being said it is a great gun if you like to practice tactical jam clearing. I've never shot a P-22, so I can't say anything one way or the other about it. It can't hardly be worse than the Sig though.