In my experience the Walther is ammo finicky. I haven't shot the Ruger.
I carry an M&P 9c, so that played heavily in my decision. I will say, the M&P 22 is usually more expensive, is larger than the SR 22 (same size as a full sized M&P 9) and is no more accurate than the SR 22. I really like it, but if you do not plan on using the .22 as a training aid, the SR 22 is probably the better choice due to price.