I understand that SSDs are practically instantaneous in speed
Yes, but only for small read/write operations.
Access times with the Kingston are about 80 times faster, transfer rates are about the same and it's cheaper so to me this is a no-brainer.
The Cheetah 15k.5 is not the current model and the sustained transfer varies between 73 Mbytes/sec and 125 Mbytes/sec which even a current 7200 RPM SATA drive can deliver. As you did not specify the usage scenarios it's difficult to give specific advice. I can't imagine an advantage for a Cheetah 15k.5 (even 15k.6 or 15k.7) for most scenarios but a big advantage of a SSD in some. If you do not own an SAS controller I would go for an SSD.