It all depends on your computer.
I just switched from SCSI to SATA.
In Win XP Dual Core 3.0 with 1 GB RAM , the computer
loads faster, Windows Explorer opens faster, manipulating
files seems to be much better.
SCSI has larger transfer rate, with 10,000 RPM drives they
are great.
I got tired paying high price for those drives, while SATA2
with 10,000 RPM are not much cheaper, but they have
much larger capacity.
While on my Win98 machine 3.0 CPU with 512 RAM, Western
Digital don't support their SATA drives in Win98.
I noticed lots performance drop from SCSI.
My old Win98 machine was 1.8 CPU with 512 RAM and
was much faster copying large files using SCSI.
I wish I didn't switch, but Adaptec are dropping Win98,
so I am switching it to IDE from SATA. One of the new
drives was defective, I have to wait until tomorrow
to do the final switch.
There is also SATA 2, which has higher transfer rate
than SATA.
Use Google Advance search and search for "SCSI vs SATA".