I miss my regular cab, if I could find a RCLB truck like mine to trade straight up I would. For me I find the extra cab room is just an excuse not to clean my truck out as often...stuff piles up in there fast. I almost never have more than a couple passengers anyways. I will admit though it is nice to have extra room for chainsaws and my toolbox when I'm working in the woods.
As for duallies....as stated before if you get the right tire size they aren't bad. My friend's C3500HD (6.5/NV4500) gets about the same mileage as my 5.7/Auto K2500. I can't remember the tire size off hand (they are on 19.5 rims) but seem to match the gear ratio pretty well (which I'm assuming are 4.56s or something like that). That's pretty impressive for a truck that size.