One of the most significant differences (besides cost
) is Gimp being unable to do 16-bit editing.
If you're shooting in raw format then you should do all your major tone and contrast adjustments at raw conversion time and save in JPEG or 8-bit TIFF format. Gimp users often use dcraw or UFRaw as their raw converters. If you're shooting JPEG then 8-bit data is all you'll have anyway.
If you want to be able to edit 16-bit data then Gimp is not for you. Maybe you want to check out Cinepaint which is a video editing software (under GNU GPL, i. e. free). It originally was a Gimp spin-off, has a user interface similar to Gimp, can handle 16-bit data, and can not only handle digital video but digital still images also.