They're a bit hard to compare imo. The FreeBSD documentation is more coherent but also more focused/narrow, covering only the base system and a few common extensions. The Arch documentation is less coherent, but quite extensive, covering almost everything. I think the Arch wiki is less good as a proper manual, something you'd sit down and read cover to cover as an introduction to the system. But one thing that's nice about it is that it's really a one-stop shop: if you want to know what some package does, how it works, and how to configure it, it's probably there. If it doesn't play nicely with Arch or needs a workaround for common issues, chances are it's documented there too. Whereas with FreeBSD you're good as long as you stick with the base system, but it's a wilderness once you venture out into ports, with documentation/tips/workarounds scattered across the web, forums, mailing lists, StackOverflow, etc.