Actually, in city and near by area it "generally" works better to 5.5' bed. While carrying 8' sheets, to a limit, both will perform equally good. In both cases you have to have tail gate down. In favor of 5.5', parking is little easier and more importantly, it will fit in most garages. On other side, anything 6' in length 6.5' bed makes life simpler, and not to mention it looks better.
I guess it comes down to the primary duty of the truck.