I vote for the 10mm for many of the reasons above...including greater penetration with less recoil coupled with a higher energy level, which also means a flatter trajectory. Great magazine capacity is an added bonus.
Bullet diameter counts for something, but not everything. I mean, I wouldn't take a .45 +P over the mid range loads I shoot in the .44 magnum, even if the .44 is "only" a .429...
As far as barrel lengths, no problem there, KKM and other manufacturers make 6 inch barrels for Glock pistols. Long slides are also available.
The ability to easily convert the G20 to .40 S&W is also a plus, since all that's required is a barrel change and sometimes a lighter spring.