One of the advantages to the .17 HM2 is that its based off a .22 cartridge, and therefore you can get refit kits to change a .22LR to a .17HM2. I think Ruger has some for their 10/22.
Just in case yall were wondering...
.17 HM2 vs. .17 HMR
I'm waiting on buying a .17 myself (until a clear winner between all the different .17's are settled), but I was wondering what the opinion of the .17 Fireball is?