Hi Craig
It basically effects read and write speed to the drive, (Although there are other factors that affect this) however unless you are recording multiple wave files simultaneously, overdubbing wave files a lot, (Time discrepancies creep in after about 4 overdubs anyway) Streaming sounds direct from the hard drive (A la Giga Samples) or moving large files then you will probably not notice much difference, so go for the cheaper 5400 but make sure it has an 8Mb buffer built in.
If your interested in speed difference, then when I changed the hard drive in my Wersi Abacus from a 5400 to a 7200 the boot up time reduced by about 20%.
Hope this helps.

Bill