What are the merits of buying SAS drives over SATA drives, or vice versa?
SAS=SCSI=manageability, especially under load and also better prefailure diagnostics and tuning capability. Spendy and low capacity/£$€.
SATA=value, capacity and adequate performance for many loads but be aware that 99%+ of SATA drives aren't designed to work 24/7/365 under duress. Also putting them under busy server workloads can dramatically affect their MTBF.
I'd recommend SATA for everything but server and top-end workstation work. You really can't beat SAS for DB work overall.
There are two separate parts to your question. Simplifying a bit, a disk consists of the hardware and the controller. Usually when people say "SAS" or "SATA" they are referring to the controller. In principle SAS is a more sophisticated protocol, though in practice for servers with up to say 8 disks there probably isn't much difference between them.
Re the hardware: disk hardware tends to come in two classes based on the seek time. Fast disks have a seek time of 3 to 4 milliseconds while the slower disks have a seek time of 7 to 9 milliseconds. (I say "slower", but 7-9ms is still pretty fast!).
In general SAS controllers are fitted to fast disks while SATA controllers are fitted to slower disks, though there are exceptions. For example the Western Digital Velociraptor disks have a SATA controller but a 3ms seek time. So when people say "SAS disks" this is usually taken to mean "fast disks with a SAS controller" while "SATA" means "slower disks with a SATA controller".
All very well, but to actually answer your question, the seek time is very important when the disks have to do lots of random access. Good examples of this are SQL Server and Exchange. If the disks are a bottleneck then SAS disks will be a lot faster than SATA. However there are two points to make.
Firstly a good controller will make a lot of difference. I use Dells and I particularly like the Perc5/i and 6/i controllers. I have several 2950s with 6 SATA disks on a Perc5/i as a RAID 5, and these are pretty damn fast. Maybe they're not as fast as 6 SAS disks would be, but they're faster than say a 4 SCSI 320 disk RAID 5 on a Perc 4/e that I used to use in the older 2850 servers.
Secondly even though SATA disks are slower than SAS, in many small businesses disk speed won't be a bottleneck.
One last consideration is that traditionally SCSI disks have been (much) more reliable that SATA, not becuase of the controller, but simply because the disk hardware was built to a higher (and more expensive!) standard. Now that you have brands like the Western Digital RE3 SATA disks designed specifically for servers I'm not sure if this is still that much of an issue.
Here are some notes from Wikipedia on the topic (Serial attached SCSI):
SAS vs SATA Systems identify SATA devices by their port number connected to the host bus adapter, while SAS devices are uniquely identified by their World Wide Name (WWN).
SAS protocol supports multiple initiators in a SAS domain, while SATA has no analogous provision.
Most SAS drives provide tagged command queuing, while most newer SATA drives provide native command queuing, each of which has its pros and cons.
SATA follows the ATA command set and thus only supports hard drives and CD/DVD drives. In theory, SAS also supports numerous other devices including scanners and printers. However, this advantage could also be moot, as most such devices have also found alternative paths via such buses as USB, IEEE 1394 (FireWire), and Ethernet.
SAS hardware allows multipath I/O to devices while SATA (prior to SATA II) does not. Per specification, SATA II makes use of port multipliers to achieve port expansion. Some port multiplier manufacturers have implemented multipath I/O using port multiplier hardware.
SATA is marketed as a general-purpose successor to parallel ATA and has become[update] common in the consumer market, whereas the more-expensive SAS targets critical server applications.
SAS error-recovery and -reporting use SCSI commands which have more functionality than the ATA SMART commands used by SATA drives.
SAS uses higher signaling voltages (800-1600 mV TX, 275-1600 mV RX) than SATA (400-600 mV TX, 325-600 mV RX). The higher voltage offers (among other features) the ability to use SAS in server backplanes.
Because of its higher signaling voltages, SAS can use cables up to 8 m (26 ft) long, SATA has a cable-length limit of 1 m (3 ft).