Go to the store, compare all the GT's and JD's, and pick the nicest most dominant one out of all of them. If they're in different tanks, which they probably will be, just compare them and decide.
That's if you can't decide what you want. Personally I think I would get a JD.
Usually I'd have a go at someone just going out to impulse buy, but I can understand the dilemna as they're the same size fish with the same temperament, you're keeping them alone, and even I had trouble deciding
i had two JD's and they just sat around doing nothing but hiding all day long. i would personally go with the GT. i have two right now and i am glad i made the switch from JD to GT