They are the same speed wise. The Double-D edition obviously has 2 fans instead of one. XFX typically has a whole bunch of models of the same card with small differences in heatsink/fan or VRM design. Really not much difference.
I would suspect the Double-D edition might run cooler. Amazon doesn't list it, but Double-D editions come with Lifetime warranty instead of 2 years if you register with in a certain amount of time. That's about the only difference that is big.