I know that with me being the resident tow nazi, this will sound funny coming from me... If you're not planning on going up much from the 8500 lb load you're towing anytime in the near future, I'd forget the dually.
I tow weights in that range all the time with an Excursion. It's got F250 springs, airbags, good shocks, dual swaybars and a steering damper. I can tow like that load isn't even back there. And when I'm done, it rides better an average 1/2 ton pickup. I can load it full of stuff in the back or I can stick seven of my friends in it comfortably.
If you were shopping for Top Guns, the dually would be a great idea. For what you have, why put up with the hassle? I'll give you this- the duals will give you more stability in a panic maneuver and you won't get tugged around in 30MPH crosswinds, but how often is it that windy?