SCSI is an aging technology (Well Hell, mechanical drives PERIOD are an aging technology!).
Don't get me wrong - SCSI is a better technology than ATA/133. SCSI really is faster and more reliable than anything out there right now - But it is also unreasonably expensive for home users and the size is limited. It is noisy, hot and trouble to setup sometimes because SCSI is not designed for the home user.
SATA is probably the future standard, meaning it will replace PATA (IDE - What is in your system right now), but there really is no speed advantage to having SATA in your system right now. The drives used for SATA are basically the same as PATA with a SATA connection.. Kind of disappointing.
The only reason I would get a SATA drive is satisfy my need to have the newest gadgets in my computer. But honestly, SATA still has some bugs that need to be ironed out before I will let it near my PC, as well as no real advantages except the cables are cool.
Here are my picks...
Compatibility: IDE
SCSI is reliable, but requires a little more thought to setup sometimes. SATA has problems - But it is still in its infancy - Once it is around for a while longer, it should be worth looking at.
Speed: SCSI
Hands down, SCSI. PATA and SATA have gotten awfully fast, but SCSI drives still have better mechanics & electronics under the hood. This gives them MUCH better transfer rates and access times more than twice as fast. Although SATA allows for more speed to be POSSIBLE, PATA and SATA are the same speed for now.
Reliability: SCSI
SCSI drives die just like PATA and SATA drives.. Just not as often.
SCSI is the best (yet old) technology available for right now, but it just isnt' practical. You are better off getting a very fast and large PATA or SATA drive.