Based on your question between the Walther and the Sig, get the Sig. You can get plenty of trigger time and practice on a gun, which is usually a centerfire. If you decide to get a centerfire kit for the Sig at a later date, you'll have a HUGE head start as far as being proficient and accurate with the gun.
A better question to start of asking yourself is what type of shooting you want to do in the long run? Do you want to stick to shooting .22, or are you aiming to get into centerfire? Do you want to shoot revolvers or autos? Any interest in shooting IPSC, IDPA, or PPC?
I would use questions like these to choose which .22 to get. You can get a gun better suited to your needs by deciding what your interests are in shooting.