The main thing Arch has over FreeBSD is you get easy access to lots of shiny toys. Pacman repos update much more frequently and the process of updating packages is much faster. If your favorite text editor drops a new version, it will be available in Arch repos in minutes/hours; it may be months before it makes it to FreeBSD ports. And things like audio, video drivers, wireless are pretty painless in Arch compared to FreeBSD.
The biggest downside of Arch is that it sits on the complete opposite side of the security spectrum. FreeBSD makes sure you start with a secure machine and anything you do to cheapen the security is your bag; most of the time it'll try to let you know you when you're doing it wrong. The BSDs in general have a really healthy culture of security paranoia. You don't get that with Arch. Signed packages have only recently made their way into pacman and I'm not sure if they're even required yet. So a lot of the time, it feels like you just have to cross your fingers and trust that the repos you're hitting haven't been compromised. And you're constantly installing software from them.
Also, occasionally you'll need to spend 5 or 10 minutes recovering from a wonky upgrade (upgrading often is better than not).
Arch is an excellent, if not secure, desktop/laptop environment and you'd be crazy to run a production server on it.