Seashore is based on GIMP, but doesn't quite have all the features of GIMP...don't ask me to enumerate what it's missing, because I haven't used either one enough to really figure it out. I find Seashore much more user-friendly due to it being a Cocoa app. It's pretty darn good for the vast majority of basic-to-mid level image editing tasks.
Seashore is sort of a watered down version of gimp. if you are just looking for basic photo edits, or microsoft paint style, go for seashore. but if you are heavily interested in photo editing or enjoy digital imaging or graphic design, go for gimp.