One of the big differences I've found is that ps is a lot better with handeling big files. So if you have to deal with 4k and 8k textures ps will be faster at doing just about everything.
As for anything else, it really depends on exactly what you are doing. Stability wise I've found the gimp about the same as ps, and although the toolset of the two largely overlap, ps tends to be a bit more polished and easier to use overall.
That being said gimp 1.4 should be out fairly soon and apparantly much has changed.