This isn't my usual area of expertise but I have a friend who asked me the same question recently when working for an IT firm in Northern California. The simple answer: SCSI is faster. It is, of course, more expensive.
The not so simple answer: For a small scale server or anything I had to pay for myself I'd go with SATA in a minute. SATA has come a long way in recent years, its cheaper and you will find deals on them more often than SCSI drives. The difference in performance and bandwidth between the two doesn't justify the price of SCSI in my mind, but if you NEED that extra bandwidth then SCSI is the way to go. SCSI as an interface has also built up a reputation for reliability. On paper both seem to be reliable options (over old pATA IDE anyhow), but I don't have any experience with SATA outside of my own personal desktop use. I would consider the following before purchasing:
What kind (if any) of RAID are you using?
SCSI drives often (not always) have better warranties than SATA drives.
If you are using a PCI controller with either you are wasting your time and money. For both modern SCSI and SATA you should use a PCIe or PCI-X controller. If you're using anything that is integrated or onboard make sure that it has full hardware support, that way you don't eat up your system resources with some flimsy software RAID. If you decide to go SATA, check out SATA II drives (II in roman numerals, the "2.0" standard has not been approved yet to my knowledge).
My friend ultimately went with SCSI himself. I have to say now I'm curious myself what other people would suggest, any takers?