OK, so what I'm getting so far is the Dually feels better.
Several have mentioned side to side motion and how a wider stance affects the stability.
Wouldn't a difference in side to side motion first be felt in the suspension? Not because of a wider stance.
If the suspensions where identical you wouldn't feel a difference until the forces were great enough to begin lifting the outside tire off the surface... then the wider stance would make a difference.
I believe those who say the dually feels more stable... but I also believe... in most situations... the difference felt is because of the heavier suspension on the dually not because of wider stance and 4 rear tires.