Keep in mind that all I have is knowledge, no experience. But. A common theme if you look around the haunts of experienced air rifle hunters is that you don't want to violate the sound barrier. Any projectile that crosses from supersonic to subsonic will get hit by its own shockwave, which will knock it around quite a bit. And since pellets have very little mass or cross-sectional density, they slow down fast. So really what you ideally want is the heaviest pellet you can throw at 900-1000 feet per second, avoiding the speed of sound, which would be the death of your accuracy.

The .22 caliber pellets are obviously much heavier than the .117 (between two and three times heavier!), giving significantly more energy at the same speed. Plus they're just plain bigger at the terminal end (0.0379 square inch versus 0.0107 square inch), which translates to more tissue destroyed and a more reliable kill. A little more expensive than the .117's, but we're still talking in the neighborhood of two or three cents per shot (it looks like the truly exorbitant 'ultra' ammo is 6 cents per shot). Not exactly breaking the bank.

I was eyeballing this rifle for myself...