I just shot a P22 yesterday. I had been thinking about getting one. Now its definitely one of the top two on my list of future purchases for handguns. I really liked it.
I've owned my p22 about six months now and have only had a couple jams, mainly because of shooting with a suppresor, and simply love it. 300 is a fair price if its new. as stated before the interchangable back grips make a world of diffrence.
I can't make the comparison because I own and have only shot the P22. It is a lot of fun, cheap ammo and fairly accurate if you use good technique at short distances. I have not had any problem with it. I think it gets more criticism than it deserves. You will enjoy it.
I've put a lot of ammo through both. I own the Mosquito and a friend of mine who I shoot with very regularly has the p22.
Mosquito:
The comments above about the trigger are right on. The trigger on the mosquito sucks. Mine started out picky on ammo, but after 500 or so rounds of mini mags and a couple bricks of cheap stuff, it eats federal bulk with no problem. Mine probably has 6000 or so rounds through it. Also, in addition to the terrible trigger, the sights are cheap and plastic. I'm really not a fan of the sights. Even with the bad trigger and crappy sights, if you work at it you can get pretty good groups out of it. It's an accurate and reliable gun once you get past those two obstacles and get it broken in.
The P22:
The P22 feels too small to me, so I don't like it. I also don't like the sweep down mag release underneath the trigger guard. I wanted to buy a gun as a training aid and I wanted it to operate like my larger caliber pistols (one of which is a sig), so that's primarily why I went with the Sig. The trigger on the P22 is better, but it's not great. Certainly not what you'd get with a Ruger or a Buckmark. But they aren't really the same kind of gun. As for being picky on ammo, his P22 is much pickier than my Mosquito now that I've gotten 'er broken in. His needs a steady diet of Mini-Mags to function reliably.
I think you'll experience the ammo pickiness with either gun until you've got a few thousand rounds through it. I think you should go with the one that fits well in your hand and points well for you, as the ergonomics seem quite different to me. Other than that, they are pretty similar guns at a similar price point, and in my experience, have pretty similar performance.
I teach with the P22's and they work well for about a year, and then I start to have trouble. No experience with the Sig, but I am interested in maybe finding a better .22 for the classroom. The one thing I hear over and over again is "It's a .22!"
The early P22's were very picky about ammo. Not so much of the later ones.
As for the price, does it have the fake comp? They run around $349 without the comp. Good luck!
I've owned a short-barrel P22 for several years. Really fun to shoot, pretty accurate and eats whatever ammo I have had laying around.
Only complaint- The safety can drop down when firing. You can google this and read about several ways to fix it. I did the filing trick to mine and it has worked fine for the last 500 rounds or so that I've put through it.
At first, I was disappointed in my P22. Wouldn't go thru a wag without 3-4 FTF. I got on INGO and took the advice of many folks and ran a few hundred CCI Singers and Mini Mags thru it. After I did this, it will shoot anything. It is now my favorite semi 22! It is quite accurate too. I like the sights and can hit a bulk size 22 box at about 25 yards consistantly. It is a really good training pistol as well. If you get one you will ike it. Just keep in mind they can be a little picky at first. $300 isn't a terrible price for a new one. I got mine for $240 lightly used. I seen them at the Crown Point gun show new at one table for $300 and at another table for $250. I almost got me another, maybe I will next time.....
I have a Mosquito and shot a P22 plenty of times. Yes, the P22 is a fair amount smaller than the Mosquito, if that would play into your decision. The Mosquito is kinda picky about the ammo you run through it, but if you use the right kind, it operates a little better, from what I've experienced. The P22 I shot just had random jams and FTEs, so it wasn't as easy to diagnose what was wrong.
I really like them both though. I am being really nit-picky about them. You can't go wrong with either.