As a collector I've been lucky enough to acquire 3 Buck 110 autos- 1 w/round activator and 2 w/the oblong activator. All are dual-action (not "double action", which would mean the knife opened AND closed automatically)...dual-action allows the 110's to be open and closed by hand, the old-fashioned way.
My experience has been-though limited- based on actual use and there has been notably *easier* action on the round button, but once the user finds the "sweet spot" in the oblong button it is a faster action with more power. Although i'm sure all three were converted by different persons the two oblongs-one which i EDC- are less likely to accidentally fire in the pocket since the oblong button does not rise above the plane of the scales much at all and only operates if firmly pushed at the end furthest from the pivot. The round button, though easier to fire, is also easier to fire by mistake since it protrudes a good 1/16th of an inch above the plane of the scales.
If you want one to use, get an oblong. If you want one to collect, or will carry it only in it's leather pouch, get the round.
Personally, i bought the first oblong on a whim and liked it so much i bought one each of the oblong and round as "keepers". All operate via leaf springs in the bottom of the knife. I prefer this to coil springs, especially on heavy blades, but i think there might be coil-operated 110-mods out there.
So far, despite being after-market modifications, all three are still Buck through and through.