If you're comfortable with Debian then you shouldn't have too many issues with FreeBSD as there is a lot of transferable knowledge between the two (FreeBSD even supports a lot of GNU flags which most other UNIXes don't).

Plus FreeBSD has a lot of good documentation (and the forums have proven a good resource in the past too) - so you're never going it alone (and obviously you have the usual mailing groups and IRC channels on Freenode).

While I do run quite a few Debian (amongst other Linux) I honestly find my FreeBSD server to be the most enjoyable / least painful platform to administrate. Obviously that's just personal preference, but I would definitely recommend trying FreeBSD to anyone considering ZFS.