I would go with the Walther P22 over the Mosquito. The mosquitos are known to be very ammo finicky, plus extra mags for the gun are $50. I have had good luck with my P22. It eats about any ammo that I put through it.
If you must have one of the two, then I vote Walther. The Mosquitoes are problematic. I also thing the Ruger Mark III, Ruger SR22, GSG and S&W M&P are great choices.