9i is not out yet but if you plan to upgrade may be 8i would be your choice since it's a stable release, as usual new products are always buggy expect the same from 9i!
9i has quite a few new features though, you can check on OTN for a whitepaper with 9i new features
I would advise you to go with 817 for the time being as its not feasible to upgrade directly to 9i ASA 9i is released. that tales 6 to 9 months to be stable version. Its lot easier to migrate/upgrade from 817 to 9i than 73 to 9i.