Sorry to reply with an RTFM, but this really is better than anything I might manage to waffle on about.
ftp://ftp.compaq.com/pub/products/servers/proliantstorage/drives-enclosures/sata-vs-sas.pdf
The best of SATA is the first S (SATA and SAS use the same interface / cables). The best of SCSI is everything else. It's just marketing really - SATA is already a buzz word, whereas SAS is just starting to be available.
************************************************Not to hijack this thread but what do you think about SAS drives? I just bought new Dell 1950 servers and am going to be using them in the Citrix farm. Dell claim they give you the best of SATA and SCSI in the same drive.