24x7 PC

Obviously 7200rpm. Unless, you add a cooler/fan for the HDD which would then make it cool enough to run. But then it depends on what you're going to be doing those 24 hours.... If you NEED the speed....

Speed ------ 10 K RPM
Cool -------- 7200 RPm

Go 4 a 7200rpm hdd.

these things does not matter
only matters is thing that proper ventilation & a robust power supply with atleast 400w