The nose section is much roomier than the upper deck. There is more room for overhead storage, and generally there is a large open area in the middle of the cabin providing even more space. The galley is also more spacious than those I have seen at the back of most upper decks.
You also have at least a small amount of traffic into and out of the cockpit on the upper deck. On the main level, the F cabin is accessed only by the F passengers and the f/a's that are working there.
Additionally, keep in mind that a handicapped passenger would have trouble going up the stairs (most airlines with Biz on the upper deck usually have a few rows on the main deck too). I worked for a medical assistance company and we frequently purchased F class seats for people with broken legs, back injuries, etc.