Does your HDTune look something like this? I know I'm posting a HDTach graph, but they should be similar:
What HDTune is telling you is that the two drives will do a sequential read faster than the SCSI drive. If you're doing something like video editing, video capture, or transferring large contiguous files, the two drives will be faster.
The advantage the SCSI drive has is random access time. The random access time for this drive (which is 15,000 RPM) is about 5.6 ms, while the access time for an average 7200 RPM drive is going to be about 13 ms.
You're going to see things like OS loads, program loads, perhaps even game loads too be faster.
Suppose you want to read 100 files, randomly located on the drive, of size 32KB or smaller.
For the SCSI drive: 100*5.6ms + 100*32KB/(75MB/sec) = 560ms + 43ms = 603 ms.
For the 2 drives: 100*13ms + 100*32KB/(75MB/sec) = 1300ms + 32ms = 1332 ms.
As you can see, in cases where you're working with many smaller files, the access time would be the biggest contributor to the total time. Flash-based drives would be at a great advantage here.