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.

Nah. Format the disk, bootstrap your root partition, edit some config files, install your packages, done. If you know what you're doing and have a fast internet connection, you're talking about 10 minutes to a working OS. Unlike Gentoo, which has a days-long compilation step.

You missed setting up a bootloader, which often is the most tricky part, sometimes due to some funny update in the freshest version of the ArchLinux which wasn't fixed yet. Bleeding edge and all.

No, bootloader is easy:

# pacman -S syslinux

# syslinux-install_update -i -a -m

# vim /boot/syslinux/syslinux.cfg # ensure path to kernel etc. is correct.

The main difference between Arch and Gentoo from an installation point of view is that Arch generally doesn't include configuration in its packages. So for every package you install, you have to manually set up the config before it will work. Gentoo often has the config set up for you. In my experience, this often makes Gentoo faster to set up than Arch, even though you are building all the packages. I actually moved to Arch, though, because the lack of configuration means that there are less dependencies in Arch (if you want optional dependencies, you have to configure them yourself). For me, this made Arch a bit more flexible. You can also build all your packages by hand using the Arch Build System (ABS). The one thing from Gentoo that I wish was in Arch is the license manager. I liked the idea that it would keep track of the licenses that I agreed to and warned me if I tried to install a package with a different license.

Really? I didn't find it too difficult, easily done in ~10 minutes. I mean, a Gentoo Stage 3 install is pretty easy these days as well, but I think Arch is still substantially quicker to get running.