zfs is more mature and has a better feature set. It's also about the best file system for a NAS, since you can detect and repair single bad blocks if you run zfs's version of RAID5/6.

btrfs is lighter on memory use and a bit easier to administer (until it eats your data). But if zfs licensing becomes cleared up and it becomes a normal part of the linux kernel that would be a lot more attractive than btrfs to me