Well, which "interface" is faster?. It really depends. There are multiple SATA interfaces available (150/300/600 MB/s) and there are multiple SCSI interfaces available. A quick scan of wikipedia tables indicate that the best SCSI interface speed is higher than that of the SATA equivivalent. But, the interface means jack **** if the HDD sitting on the other side sucks.
So your question should be posed in 2 parts: interface type AND hard drive type.
EDIT: when I said 600 MB/s are available, I really meant WILL be available
. As far as I know the highest SATA in the market right now is 300 MB/s interface.