Have read this board many times, but just finally joined. Been shooting nearly 50 years, have many toys, and have owned many more no longer in my possession, including many 22s. Had a few thoughts.
First, I just picked up a GSG Firefly recently. I never owned the Mosquito, I read the bad reviews, but when they essentially re-launched the same gun w/out the Sig brand for over $100 less, the equation changed for me. It cost me $210 new, all in, with a threaded barrel no less, which is less than all the other choices.
I generally agree that the SR22s use better slide materials and cycle more reliably than the other choices on average, and compares favorably to the P22 by a wide margin. I have owned both. I personally like the Buckmarks & more classic Ruger 22 pistols (my first was the original Standard), but they are a little heavy & harder to clean, and they generally cost more, so the SR22 is a good choice.
That being said, I've now put a few hundred rounds through my Firefly without a failure, and it is fairly accurate. I've run standard and high velocity, CCI, Remington Golden, Federal, and Agulla ammo. No hyper vel yet. The one thing they all had in common was 40 grain, which may be better than lighter weight bullets in this gun. On the downside, the trigger is not great, the decocker and safety are flimsy, and I'm not a fan of zinc alloy used in the Walther and the Firefly/Mosquito, though it is probably adequate for 22 pressures. Not a big fan of plastic frames either, which are widely accepted. I'm not sure if I just got lucky, got a good one, or GSG made improvements before re-launching, but for $210, my Firefly seems to have been a great value so far, and costs less than the others. I really like the look & feel (still Sig-like).
On a side note, I realize this string is about combat style 22 practice pistols, but I feel compelled to point out if you just want a trail gun, plinker, or small game hunter, the revolver choices run circles around all of these. In a centerfire, there is lively debate between the advantages and disadvantages of revolvers versus pistols. Revolvers offer better reliability, more bullet configs (no feeding), some more powerful rounds (357), differing aesthetics & ergonomics (subjective), but give up a lot on capacity vs a double stack 9 (less so on singe stacks). Many centerfire pistols feed fairly well though.
This is all well known and widely debated, but when it comes to rimfires, the revolver reliability advantage is 10X as large (since rimfires do not feed as well), and the pistol capacity advantage pretty much goes away (most semis are 10 rd single stack & revolvers come in 6, 8, 9, & 12 round capacity).
Revolvers generally cost more, Smith Kit Guns, Ruger's 22 SP101, GP100, Single Ten, even their new LCRx 8 round 22 is in the $400s. That said, you can find a used Taurus 94 in the $200s (9rds), same for the Rossi version, and some older 9 round H&Rs can be bought in the $100s. Something to think about.
Hope my first post wasn't too wordy or far off topic