I just went from a 255 BFG Mud to a 285 BFG A/T.
My cliff notes on the sizes:
1. The amount of trimming you'll need to do is significant, and spacers or less backspaced aftermarket wheels are necessary with the wider tire. I have sport flares that were already trimmed for 255's, and when I switched to 285's I noticed some areas that I had to hack more of the inside of the bumper flare. Not really a big deal though. The UCA and pinch weld are more of an issue with clearance, IMO.
2. The 285's look MUCH better to me, especially since I'm running a 17" wheel. 100% subjective, as mentioned above by someone else.
3. Availability of all terrains is almost non existent in the 255 and the price is higher.
If you're ok with the price difference and limited selection, go for it. They require less work and cost to make fit (no spacers or aftermarket wheels necessary). I run spidertax so the UCA rub was never a concern, but the pinch weld area and front bumper/ flares are.