BTRFS is clearly designed for one target application -- massive, flexible storage. Think of server farms and cluster computing. I don't think it offers any advantage for running an OS -- it is not faster than ext4. It offers multi-device performance very similar to RAID, with no special controllers or drivers, so that makes life simpler. For my 2-drive BTRFS filesystem, I used the default options, which sets metadata to be mirrored (similar to RAID 1) and data to be striped (similar to RAID 0). I have not done any benchmark testing -- you can read in Phoronix and elsewhere how the performance compares to ext4. I have about 750GB of music, videos, images, and docs in my /mnt/DATA filesystem, and it has shown no problems in 18 months. When I first installed it, there was no defrag and no fsck available. The first time I ran scrub (the btrfs version of fsck), it found correctible errors on 13 inodes, and fixed them, and there were zero non-correctible errors. It has never found any more errors, as you see above. I worked through a full defrag one time, which is a 2-step process, and that went fine. But I kinda think it was not necessary, similar to ext4.
I have never tried ZFS -- I can't comment on that one.