Seems to me that you ought to consider the disk drive itself too, rather than just focusing on the interfaces (which seem to be fast enough??). As you probably know, SCSI drives are currently the fastest platter spinners -- spinning at 10,000 or even 15,000 RPM as compared to 5,400 0R 7,200 RPM as in the case of the current group of ATA/EIDE and external Firewire drives (and I assume USB2 drives also). The extra revolutions usually translate to faster access/reads/writes and unfortunately more expensive drives with less storage space.
For what it's worth:
I have a new 2 GHz P4 workstation that is running both Ultra160 SCSI and Ultra ATA 100 drives. According to my Matrox Disk Benchmark program, my two new 10,00 RPM Ultra 160 SCSI drives (18 GB and 73 GB) each have a minimum/average/maximum write capability of about 23/27/59 MB/sec and a read capability of about 23/27/39 MB/sec. My new ($90 backup) 80 GB 5,400 RPM Ultra ATA 100 drive is just a little farther behind -- it has a minimum/average/maximum write capability of about 13/27/39 MB/sec and a read capability of about 14/21/29 MB/sec.
Note, that according to my humble benchmark (which I know nothing about) both the Ultra 160 SCSI and Ultra ATA 100 interfaces are fast enough for my disk drives.
Big price difference however, my 73 GB SCSI drive was $800 -- my 80 GB ATA drive was only $90! I'll let you do the math, as I'm affraid too.