Re: Which one?
I had the Walther and sold it for a Mosquito.
My take was that the Sig was of much better feel and quality than the Walther. However, the Walther had better magazines than the Sig. Although the Sig feels different than its larger 22x series cousins, it certainly felt more like a real pistol in my hands. The Walther felt more like a toy.
The Walther now belongs to a friend of mine. He isn't the best at gun care, but has in no way abused it. It has since fallen apart. The muzzle weight won't stay in place. There is either a FTF or FTE about every other round. I could go on and on.
The Sig has held up well, but i'm still not wild about the plastic frame and aluminum slide.
If you have the money, and want a combat pistol in .22lr, do the conversion unit route. Otherwise, go with the traditional .22lr pistols for plinking (Ruger MkII, Colt Woodsman, Buckmark, S&W 41, etc.)