Hi,
My 2 cents - use a .22 for HFT if it is going to be your only air rifle and you want to do pesting or hunting with it also. If you are shooting American Field Target (Hunter Class) you are limited to 20 FPE. Typically, folks use AA or JSB 13.4 gr .22 pellets shooting them around 820 fps. They can be very accurate, though the trajectory will not be as flat as a .177 (unless you are shooting 13.4's in .177 of course).
I have been pretty successful in Hunter Class using a .177 shooting at 820 fps. The hard part of FT is not getting your dope figured out, it is ranging accurately and reading the wind. I have shot some really good 30 and 40 yd cards in the AGN 30yd challenge using the .177 at 820 fps. In fact, most of the shooters on that thread who are shotting the 13.4's are at 820 fps or lower, and there is no requirement limiting speed. They just seem to shoot better there.
Cheers,
Greg