You are looking at 11.2" wide for the 285 and 12" for the 305. Like kjeeper said, you will take a bit more of a mpg hit with the 305s due to more contact (friction) with the road.
I was torn between the same 2 sizes. What ultimately made me go with the 285 was the fact that it was a load range D tire. The 305 was a load range E, which I was trying to avoid.
Here is what the 285/75/16 will look like on a 4.5" backspaced wheel. Ignore the rabid attack squirrel / recovery device.