Yup, very much depends what you do. Sequential reads or writes a 7200 won't be much faster, but if your disk is rather fragmented, or you are accessing lots of small files, then 7200 will be a bit faster.
Most of a disk's IO time when accessing small files is rotational latency (i.e., waiting for the sector of the file to rotate into position under the drive's head), and a 7200 is say 30% faster in that respect.
But.... that is considering equal size drives.
A 5400rpm drive that is 1tb in size will probably be faster than a 7200rpm 500gb drive because the first half of the disk is faster, and on a 1tb drive the 500gb worth of files will all be in the first half of the disk.
If you're willing to get a bigger drive and keep it under 50-60% full, it will be faster than a drive of the same speed but near max capacity storing the same amount of data.
see here:
http://macperformanceguide.com/Storage-WhyYouNeedMoreThanYouNeed.html
Given that 5400rpm drives are generally larger, the difference isn't so clear cut. If you're looking to keep a 7200 rpm drive with heaps of free space, go for it. But if you're going to be pushing to fit your data into a 7200, and a 5400rpm drive will give you a lot more free space, the 5400 may well be faster.