There is a big enough difference (30+%) that you will notice it under certain circumstances.
If all you are doing is web surfing, office documents, general stuff, 5400 rpm drives are fine.
If you are doing photography or video editing, go with a 7200 rpm drive. Photoshop, Aperture, iMovie, Final Cut, etc. are all resource intensive (well, actually, it's the underlying media that are resource intensive) and you will definitely see an improvement in overall performance with the higher speed drive.
Bottom line, if you are comparing those two models, there's more at play than just the difference in the hard drives. For the extra $300 you are also getting a much larger display, the ability to add more RAM in the future, and a more powerful graphics processor. I suggest buying with longevity in mind, and for me that would dictate the 27" iMac.