Ken,
If you have not towed a heavy fifth wheel or any heavy trailer before without a dually then you would be in for a shock if you did. I have owned and towed with both and would never go back to a SRW now after having towed with a DRW truck. The dually's have better stopping ability with 50% more tires on the ground, better stability while towing with a wider wheel base, less swaying and being shoved around in high winds or passing other trucks, and as you mentioned, better towing capabilities as far as weight in the bed.