I've used both, and I recently switched from Gentoo to Arch when I built my new desktop last month.
While I prefer the way Gentoo does certain things (slotting is wonderful, eselect is the best alternatives system I've seen, eselect news beats checking Arch's website for breakage warnings, and Portage overlays beat the pants off of Arch's pacman repos vs. AUR dichotomy), I'm sick to death of compiling everything. I actually got my Arch system up and running in a single night. Gentoo would've taken the whole weekend.
Also, if you forget to update for a while, it's much easier to drag an Arch system into the present day than it is to do the same with a Gentoo system.
While I prefer the way Gentoo does certain things (slotting is wonderful, eselect is the best alternatives system I've seen, eselect news beats checking Arch's website for breakage warnings, and Portage overlays beat the pants off of Arch's pacman repos vs. AUR dichotomy), I'm sick to death of compiling everything. I actually got my Arch system up and running in a single night. Gentoo would've taken the whole weekend.
Also, if you forget to update for a while, it's much easier to drag an Arch system into the present day than it is to do the same with a Gentoo system.