The hard drive that spins faster is going to have better speeds, and I'm sure you get this by now...

However I wanted to point out that the speed the platter spins at is not the only things that matters, things such as the transfer rate (like IDE and ATA) and buffer size also play a major role. For example you can have a hard drive with a 5400 RPM and 16 MB buffer and a 7200 RPM with 2 mb buffer. So if it is something like this the gap between the difference of speed will be less... you will notice a big difference, but not all that much.

Apart from speed, I think the sound is different as well, because my friends laptop (which has a 5400 RPM drive) sounds different than my pc (7200 RPM).