My experience with 6x6's is that singles go through snow and mud much better than duals. Other comments here abut the value of chains are right on. They make a huge difference, but are a pain in the posterior to put on, and you do not want them on all of the time. If you run much on the road, you are wearing them out for no reason, and they are tough on tires and suspension. The best situation is to have the 1100 x 20 tires on the correct hubs to make them track properly. However, on my old M220 GMC duece, I just pulled the outer wheels on the rear axles for winter. The tracking difference never really caused a problem. Lowering air pressure so the continuous center rib on the military tires could move up allowing the outer treads to grip helped a lot. I went through soft snow up to 18" deep, but again snow can be powdery or very hard, making a big difference in how the truck handles it. I got stuck a couple of times, but usually I should not have been where I was anyway. Singles are definitely best in mud, even with chains on duals. Duals in some conditions push too much stuff ahead of them, severly hampering your ability to move forward. Once the center of the dual sets pack solid with mud, you are done, period. When it comes to ice, chains are the only really good answer. I have two complete sets, but after a couple of experiences of struggling with them, both putting them on and removing them when they are packed with mud, I shy away from them unless I absolutely HAVE to use them. If your truck is used a very large percentage of the time off road so you can put the chains on in the fall and take them off in the spring, then they are a more attractive answer. I will witness, though, that with them on, you feel almost unstoppable!!
That's why I went with the NATO NDT's on the Beast.
Yeah, NDT's 
