While I've done very little with Gentoo, Arch's packaging seemed to me like running OpenBSD's ports tree from cvs, but without the option of an updated-every-six-months super-stable milestone release. It's inherently a "bleeding-edge" distro, which is not necessarily true for OpenBSD. (I run OpenBSD from snapshots/source and update a few times a month, but I also maintain some ports.)
I've only experimented with Arch, though, and I'd also like to hear from others who know both well.
I've only experimented with Arch, though, and I'd also like to hear from others who know both well.