I've never really understood the difference either.
Are quads sweeter? Gulden Draak is pretty sweet.
Are their attenuation %'s lower? I think Southampton Abbot 12 and Gouden Carolas Grand Cru of the Emperor are lower than La Trappe Quad, which is only slightly lower then St B Abt 12, which is quite a bit lower than say Rochefort 10.
Fruitier? Rochefort beers are quite fruity. La Trappe comes across as different fruit, as do the Unibroue beers listed in 16E.
Lower carb or fuller body? Don't have enough spare liver cells to open all these simultaneously to compare. Perhaps a side project for NHC attendees to taste a bunch of BSDA/quad together?
I dunno...I'd like to see them all together honestly.
I don't think I'd enter a strong dark belgian in 16E unless I had created a good clone of a well-known beer already listed there, or had created a Christmas-y/heavily spiced BDSA. Just my two cents.