If there is a feature/fix in 9 that you will be wanting/needing, then I would start with 9Beta and just set up for testing until release. If not, go with 8 and then upgrade down the road if needed.
FWIW, I'm running 8.3.1_P2 and 9Beta, but I have a simple UFS set (5 disks of various age/config) with a minimal # of users (5).