If you compare a 235/85R16 to a 265/75R16 both E range tires, they are about 6 pounds different weight wise. Depends on the brand of course. From my experience, the 265 tires are much better road wise. Great steering tire and you can corner way faster than you should. The 235's are noticeably better in the sloppy stuff. Slushy snow, mud and dirt in general for me. I never noticed an significant MPG difference. Pricing wise, the 235's are usually cheaper and in general I like them better. They aren't as good in rain in a more mud terrain style but that's not a surprise.
Cheers,
Jorge