Oh and in response to the "it's more expensive to buy tires for a dually". I don't really think this is true for 2 reasons:
1. The rear tires on a dually last much longer than a single rear wheel truck because there is half the load on them. A good quality set of rear tires will last almost twice as long as the front tires
2. Newer single rear wheel trucks take ridiculously large tire sizes such as 18" and 20" that are very expensive. Newer dually trucks typically take 17" tires which are much more reasonably priced.
Short of the obvious advantage of still having one rear tire on a side in the event of a blow out, a dually also has the advantage of being able to run much lower rear tire pressures than a single rear wheel truck resulting in a much better ride. I have also noticed that the rear tires tend to run much cooler on a dually.
Just my 2 cents worth after having both types of trucks!