What is the collective wisdom on the better of the two for function, reliability, and ergonomics?
Sig Sauer Mosquito, or Walther P22 (short barrelled versions)

I have owned a P22 and have not used the Mosquito. But what I hear about the Sig is not good. It is more like a real full size pistol, and the compliments seem to end there.

Now for the P22. It has a number of little problems. A shame, as with a little more quality it could have been very nice. Trigger is not too good double action, but is okay though not great in single action. The barrel nut will give trouble coming loose after some use. Loctite usually fixes that. The safety gets to where it comes on by itself after a couple thousand rounds. The P22 can be ammo picky until broken in by a brick of ammo at least. Mine was pretty reliable after that.

Now the size and fit in your hand of the P22 is very nice. The balance of it is nice. That is why I say it could have been great. Needed a better trigger and better quality. The design is alright you just get the feeling they tried to do it too cheaply without quality materials.

I see two better alternatives. Either a Ruger or pay a bit more for a Browning Buckmark. The Buckmark is one fine pistol. The Beretta Neos is also a better choice than the two you ask about.

I went through the comparison of pistols when I bought mine, looking at the P22 and others, and didn't like the trigger on the P22. I also didn't like the Ruger's disassembly, and the trigger on it was not as good as the Browning Buckmark, so I bought a Buckmark. I've very happy with it.
I also read many adverse criticisms about the Sig Mosquito, so I did not buy one of those. I did look one over at our gun store though, and liked its compactness.

Don