285's have the advantage of a little more grip and floatation but this comes at the cost of weight (particularly rolling weight) which leads to more wear on components like brakes, axles, gas mileage, and the inevitable back that has to lift the tires.
255's have the advantage of cutting through crud like mud with a solid bottom and being lighter.
The difference in floatation/grip are there but minimal--you could argue it, but after driving both tires in Colorado and Moab, I did not notice that much of a difference.
I also prefer the look of the thinner 255's, so that is what I went with. Either tire would be appropriate and awesome.