I saw difference enough that I would never go back to 5400.
Good advice, and if you can keep large amounts of data on external drives to off-load the internal disks you can get the best of both worlds.part of the speed difference people perceive when installing a new hard drive is simply because the disk is unfragmented. the other part will be from the actual hard drive performance, whether it be from having more space or a faster rotation.
in any case, go for space first, speed second, unless you don't have store much data in your computer. a half-full 7200RPM drive will be slower than a larger 5400RPM drive with the same amount of data.