I agree that SCSI Ultra320 is faster, but I disagree that it should be used for backup for a bunch of reasons...it's usually used in a RAID Array for high speed audio / video work. As a backup medium it's not practical at all because you need a separate controller card (which leaves out laptop owners) and the drives are not "hot swappable".
A FW800 Array is still my choice for work AND backups. Inexpensive / fast / universally accepted / hot swappable (on Macs, anyway

). It's twice as fast as FW400. Speed isn't the only difference between FW400 and FW800: FireWire 800 also allows longer cables, which can dramatically increase the distance between FireWire devices. SCSI...can't (or really shouldn't) go more than 6 feet in length.
When life gives you dilemmas...make dilemmanade.