Target shooting: 17 HMR
Hunting: up to 75 yds 22mag, past 75 yds 17 HMR
Cost: 22 mag
Avaliablity: They are equal in most areas now
Accuracy: 17HMR by far!
Plinking: 17 HMR, though the 22 LR seems to be the obvious choice
I have to admit that I have very little experience with the 22 mag but I did a lot of research before buying my 17 HMR. I too looked at all these factors before deciding which I wanted.
I hunt lots of small game with my hummer. Crows are my usual prey. I have taken many as far as 250 yds, the 22 mag just isn't capable of such accuracy. I've also taken coyote, squirrel, rabbit, porcupine, coon and beaver with my HMR. When it comes to targets and long range plinking (200 yds) the 17 HMR is so accurate and predictable that you won't want to put it down lol. U'll just keep moving targets further and further away until you cant hold the crosshairs on a pop can anymore.
The 22 mag is about $10 per box while the HMR is about $16. Personally I think the extra cost is worth it.
Ultimately I'm very happy with my hummer.