Duals without a doubt, you can PM BEH on here and he can attest to it. I was pushing a 9750sts with a 635F on 18.4-42 Duals right next to his MF 8780XP with 30ft head on 30.5-32's and was able to drive straighter and go places his combine wouldn't.
Aside from mud, another benefit of duals is if you have a flat or tear up a tire, one 20.8 is going to be cheaper to replace and/or find then a 30.5. That is if you luck out and only blow one tire and not both.
look at the big 1050 or 1100 singles.... I think it depends on your crops and row conditions. Some of these corn hybrids that went late in 09 or just harvested in early 2010 had really sharp stalks and torn up a ton of tires. I also like those big 620 duals.
If on 20 inches I would go with super singles, if on 30 in get the big duals
Here, central LA, you rarely see duals. Maybe in rice country in the southwest prairies of the state where they have 10" of sand and a clay hardpan.
Otherwise duals here are a recipe for disaster, unless you wide space them with picker spacers and put them on 36-38" centers. They pack and will not go forward. In heavy sticky clay ground big singles anyday. 900 radials R-2 or the big floaters with 28L's R-2 in the back.
Next, tracks, rubber or steel.
Every 6R picker with duals are either wide spaced or have scapers between the tires.
We put 18.4 42 duals on ours this year and was shocked what we could go through compared to singles. gotta have rear wheel assist with it though. Two neigbors seen what i went through and are planning on getting duals for next year.