This question may be moot now. Oracle EOL'd FS1 and RIF'd most of the FS1 staff a while ago. While both ZFS (now ZFS7) and FS1 are/were unified storage systems, ZFS is primarily NAS; FS1 was primarily SAN with some QoS and multi-tenancy features and such.
Ability to speed application performance.
With massive multi-PB data stores today, I believe the most important thing is how well the system can scale.