I'm leaning toward a 9" like you mentioned the availability of parts and the ease of gear changes not to mention my 2 racing buddies I run with are ford guys and they have a narrowing jig for the 9in and an abundance of parts.
Another plus is if you happen to break one at the track most of the time there is someone there with an extra third member to loan you. keep in mind this is not a good idea if you run a 727 unless you take it apart and inspect the sprag.
