I've used the first generation FAST and the BS3 on my two projects. They both work pretty well, but the BS3 definitely has some nice features compared with the old FAST (like customizable axes on the fuel/spark tables). The new XFI looks very nice as well, and the software documentation (online at the FAST site) looks clearer than the BS3. Also, the new XFI has switchable calibrations, which would be nice for pump gas/high octane tunes, etc.
But if I were on a budget, I'd go with the 950, which has worked well for lots of folks. There's very good knowledge and support on the chevy talk forum, including a very responsive Holley engineer (Doug Flynn).
http://www.chevytalk.org/threads/pos...=0&Board=UBB12