I would say OpenSolaris. Solaris 10 can be a pain if you dont know Solaris. OpenSolaris on the other hand is very similar to a modern Linux, like Ubuntu or SuSE. Everything gets detected automatically and setup.
And, you can run Linux (with kernel v2.4) in a virtualized zone on top of solaris. Each zone has it is own IP adress. Each zone is created in one second, and deleted in one second - this assumes ZFS. Each zone takes very little overhead, like 50 zones take 1% cpu utilization or something like that. Each zone takes 40MB RAM and 100MB disk space. I can recommend linux zones aka Brandz and solaris zones - with ZFS. Neat! This message posted from opensolaris.org
