Even with a single-shot I'd go with the Hornady. It's a little shorter than the Ackley version, I'd guess because to allow it to work better with plastic-tipped bullets in standard Hornet magazines, so has slightly less powder room. But the difference in potential velocity is so inconsequential it can be ignored, and Hornady factory ammo and brass is cheap, abundant and very good. I've converted .22 Hornet cases to .17 Hornets, and it's something of a PITA, though exactly how much depends on the .22 Hornet brass.
In my experience Huntsman's exactly right about sticking with 20-grain bullets for PD's and the gophers you hope to shoot someday.
