There's a big difference, though - Arch Linux has no concept of partial upgrades. You have to update, or things break. Gentoo allows you to stick with old versions and has a pretty sophisticated mechanism for that ("slots"). It's also complicated, which is one of the reasons Arch Linux is easier to use.
This makes Arch Linux unsuitable for pretty much any "production" scenario. Puppet version updated to 4? Tough luck if you didn't scramble and update all of your code since version 4 has backwards-incompatible changes.
Works pretty well for desktops - if something breaks, you can usually just fix it. But it does not scale.
Gentoo, on the other hand, has an unstable and a "stable" channel and you can use it for production systems if you've got the necessary manpower to essentially build your own distro.