Well, btrfs is a new filesystem with a completely new implementation, and it is still not in "production quality". It's in the mainline kernel, it works (I recently converted my / to btrfs), but still I had to check an option labeled experimental with a large warning while configuring the kernel.

On the other hand, this is just a port of actually working and stable FS already used in production with Solaris (and mabye FreeBSD?). Btrfs won't be ready until next few releases.