If your new fiver will be heavier than your current toy-hauler, my gut feeling woud be to go with the DRW. I've seen many SRW trucks pulling fivers and have pulled a trailer myself with a SRW. You couldn't pay me enough to go back to a SRW. The DRW is much more stable.
Dealer mention loss of milage - if you do, it is small enough not to worry about. As you see in my signature, I drive a F350 Dually, and have a close friend that has a F250. Same engine and transmission. He has 3.73 gears to my 4.30, and we are virtually neck-in-neck in MPG in the city, he has me by maybe 1/2MPG on the highway as his engine is turning a bit slowier at 65MPH.
Tire expense, I have to agree, you will have 6 vs 4, but then again, you don't replace tires every week either, so to me it's a non-issue.
I'm not sure what the dealer is talking about concerning driveabliity issues - ya, a DRW is about 18" wider than a SRW, but you learn to compensate. I have learned to back in to most parking spots, and if at all possible, I park where others don't (and have lost 50+ pounds walking from the outskirts of Walmart parking lots). In town, it drives just the same. Most roads are at least 10' feet wide, and a DRW is just under 8', so you learn to keep to the center of the lane. The only issue I have with a DRW, and after 1+ years, I still have on occasion - I turn to short, and the outer wheels run up over the curb, the only thing that hurts is you ego.
You mentioned you want a 4X4. Make sure that it doesn't sit so high that you don't have the proper clearance between the bed rails and bottom of the trailer overhang - I believe you what a minimum of 6".
There are actually 3 figures you need to be concerned about. GVWR of the truck, GVWR of the trailer, and the combined weight of both so you don't exceed the GCWR of the truck - example Truck: GVWR 13,000Lbs, but weight curb weight + traveling items (wife, dog, tools, 5w hitch, etc) weights just over 8,000Lbs plus 2,205Lbs fiver pin weight still brings me under the 13,000 for the truck. Trailer: GVWR 13,900Lbs, but I weigh only to make sure I don't exceed the GVWR of the trailer, not for calculations. So 8,000 for the truck and 14,000 for the trailer comes out to 22,000 which is under the 26,000 the GCWR of the turck allows.
Larry