I agree, areal density plays a good role in transfer rate and rotational latency. I don't think any <9 SCSI can hold much of a stick to large, new, 7200 RPM IDE.
I personally don't see the need for SCSI outside of servers. For a single user desktop, I've never seen a difference between high perf SCSI and IDE (other than cost and capacity

SCSI is nice in a server though becasue it supports command queing to service multiple simultaneous requests more effecitently.