SCSI can (depending on which particular SCSI) provide you with more devices per controller without sacrificing (any noticeable) performance. If you need to shove a ton of drives into one server, this will add up quickly. Since you are talking about RAID 0+1, depending on how much storage you are shooting for, this may be a strong factor (but you may be able to skin by on the 4-6 SATA ports you'll find on most mobo's).
SCSI is more mature. So drivers are likely to be more robust, more efficient, and more stable than those you'll find in your garden variety SATA.
You'll typically find that under heavy load, SCSI performs better. Again, this is mostly due to so called "market segmentation" schemes, but that is why you pay more. If your users are going to be mostly dealing with the usual, periodic saving of word processing documents, spreadsheets, and a couple of light media files - you probably don't need to handle really heavy loads. The RAID controller will eat the peaks of write demand in cache (if you get a decent RAID controller - see later), and you should have fairly smooth performance. Then again, if your users are constantly running large installers (development test environment) or working with large remote files - you should really go SCSI.
All that said: I think you would be served best by investing in a better RAID controller rather than investing in top of the line drives. The RAID controllers they integrate on to most motherboards are crap (for what you are trying to do, desktop use - meh). You want something with a ton of cache, and good management soft/firmware. If you buy a real server class motherboard, you may get a better onboard RAID, but however you go about this - pay the most attention to this detail. Unless you really need low latency for high demand, random access applications, top end drives probably won't give you much over the usual network latencies.