As others have mentioned, Btrfs is still experimental, so you probably don't want to rely on Btrfs just yet.
People generally seem happy with ZFS on BSD, but you might also want to consider running ZFS on Solaris.
The BSD port of ZFS is probably the most mature port, but it still lags a few versions behind ZFS on Solaris, so you won't get all the latest features right away. As of FreeBSD 8.0, only ZFS version 13 is supported--so, for example, you won't get deduplication (which was introduced in ZFS version 21). You also won't get iSCSI with FreeBSD 8.0.
If you want the latest ZFS features as they're introduced, you may want to opt for OpenSolaris or Oracle Solaris, or Nexenta (a Solaris derivative with a GNU userland).