I was making the same decision about a year and a half ago ... or about that....
Here's the results and my final decision:
The early Walther P-22's had some problems with slides and magazines..... (hey, I'm just the messenger, don't blame me!!). Newer ones, maybe since about 2007/2008 or newer seem to have those problems fixed. For any of the people that own them that I have talked to at the range, they love them.... I enjoyed shooting it, and if I ever need another .22 it will be on the list.
Research on the ISSC m-22 was mixed. When I was looking, it hadn't been on the market long enough for a good, long term review, and my local shop was selling it for almost the same price as a new Glock, so I passed.
The Sig Mosquito had mixed reviews when I was looking.
Rugers have a great reputation, but I didn't like the "feel" of it. That is not a slam in any way.. it is a subjective evaluation.
Ended up buying a Browning Buckmark in stainless. Have really liked it. Purchased for less than $300. Grips feel great. It has a "big-gun" feel, and while it does have a safety (unlike Glock/SA XD's), it does what I need it to do with trigger squeeze, sight alignment etc. I'm at well over 1500 rounds, (no clue how many at this point), and the only failures were with a box of Remington that had multiple failures. Now shooting everything except and having NO failures of any type.
For price, feel, and purpose, the Buckmark fit the bill perfectly.