Solaris: ZFS. Less power management. UPnP media servers act weird. Power draw with one spun-down hard drive: 50 W. Needs 2 GB of memory for ZFS not to do weird things (like pause rsyncs while writing).
Linux: No ZFS. 35 W power draw, one hard drive spun down. 512 MB of memory is probably sufficient.
FreeBSD: ZFS, but it's experimental.
ESXi, Linux + Solaris: 46 W power draw (no hard drive spindown capability, so that 46 W is with one spinning platter, and my full setup would probably be at least 62 W). Needs more memory (= more power consumed).
Linux + Solaris in VMware Server: No idea how it cooperates with power management, because you can't export raw devices in VMware Server.