I've done both, and they're both pretty easy if you know what you're doing.

The only thing that's a bit more of a pain in the ass in Arch is mirror selection: Arch likes to use really shitty mirrors by default, and I have to waste time commenting out most of the mirrors in the file if I don't want to be stuck with <100kbps downloads, while Gentoo is a lot smarter when it comes to picking mirrors.