Melanie: With PS, you can work in different color modes, there's color management (not sure about gimp here) you can work in 16-bit as opposed to 8 bit (there are branches to gimp like cinepaint which do this), and a bunch of actions, different and more complex tools, etc. The spot-healing brush in PS is a godsend when it comes to dust and scratches on B&W scanned film.
The interface between the two is a bit different, GIMP is very mouse/right-click oriented -- if you don't have it already, get yourself a two button mouse + wheel for your apple.
GIMP is very quick and efficient in doing stuff specifically for the web (beyond photo editing). If you intend to print at the snooty-high-quality type levels, PS will be better. In other words, I find GIMP to be an effective tool for everyday, low-pixel count work, whereas PS for the high resolution work.
Hope that helps 🙂
Jano