Admin with mostly Macs. Need server advice.

I work at a company that is pretty much all Mac, except for accounting and some specialty machines that crunch big graphics files. Our company has grown over the years and it's way past the time to modernize this ad-hoc piece of shit network.

What's the best way to go? Linux open directory, or OSX server? I'm not really considering Windows server because a] few windows machines, and b] cost.

I'd like to be able to take advantage of the ZFS file system, so FreeBSD, Solaris, and OpenIndiana [fork of OpenSolaris] could be possibilities as well.

Please chime in with your opinions.