Depends on the target application (Score 1)

The choice of one over the other depends on the application. For a desktop environment, OpenSolaris is not very user friendly as it is derived from System V. FreeBSD, especially with the ports collection, can be used to create a very customizable desktop experience on par with Linux, and the BSD flavor of the OS interface provides for a much more navigable filesystem and user interface than OpenSolaris. If ZFS is key to your application, then OpenSolaris is the way to go, I doubt that even recent forks of FreeBSD have the most recent capabilities that have been integrated into the ZFS project (real time storage deduplication, pluggable storage modules for iSCSI, FCoE, FC etc). You would be hard pressed to find anything that can rival OpenSolaris on the storage backend, FreeBSD (or any other OS for that matter) doesn't get close in the storage realm. Performance between the two is probably negligible given recent performance enhancements to FreeBSD. With OpenSolaris you have Zones for virtualization, in FreeBSD jails - both similar concepts but implemented differently. Licensing would be something to look at as well. The BSD license is truly open source, meaning that you could create derivative works from the FreeBSD OS and rebrand it as your own product with no attribution back to the FreeBSD project. OpenSolaris has a more restrictive license than BSD so if the final product is say an appliance or a turnkey VM that includes the OS, the BSD license would be much more amenable to redistribution and rebranding as opposed to pretty much everything else out there.