.22 is for killing stuff, or knocking down reactive targets like FT or Silhouette.
.177 cal is for punching paper, or be'n mean to beer cans.
That is really it in a nut shell, but here is a bit more :
The .22 cal will always be more efficient and carry more energy to the target then the .177 given the same gun with
same power plant and barrel length, etc.
The 1377 pistol is a perfect example and really magnifies the disparity.
With a 12" barrel and full mods, I get this pistol to a guaranteed 12 FPE with less then 15 pumps
( usually get closer to 14 FPE with that many pumps ).
In .177 cal, and the exact same mods, same 12" barrel, I can't get it over 10 FPE.
Furthermore, the .22 simply punches a bigger hole in the target, animal if are talking hunting....and really you don't need more power to punch paper than it takes to get the pellet to the paper....so FPE only matters for hunting.
Not only does the larger pellet make a bigger hole, it carries it's energy much further down range and delivers it with much more authority than the little pill does, and penetrates further.
as for pistol barrel length, I prefer 12" for looks , balance, and power.
To look at it from the stand point of a powder burner's perspective, He would choose a slow moving 50 cal to take down a buffalo, not the way faster .223 , and for the same reasons pointed out above.
Pacific Pellet Guns