Physics professor Rhett Allen, blogging at Wired, suggests which way you should choose in the age-old question, "Would you rather fight a horse-sized duck or a hundred duck-sized horses?"
Dr. Allen uses physics and known properties of the size, density and mass of horses and ducks to arrive at his answer. In brief, it depends on what you mean by "horse-sized." If you mean the duck has dimensions equivalent to those of a horse, then you should pick the horse-sized duck, because in all likelihood it wouldn't be able to walk very far or very fast and would not be at all agile in a meleé. It would be too big for its two legs to carry it well.
If you mean a duck that weighs as much as a horse weighs, you get a very different scenario: Your duck is the size of an ostrich or an emu, which move pretty darn quickly and are highly maneuverable, giving it a considerable edge over you in a fight. Thus you should attempt to placate it with a very large mound of whatever it is that ducks eat that has the same effect as chocolate does upon irritated female companions or family members.
So Dr. Allen would pick the hundred duck-sized horses, and so would I. Especially if we were near a pond; horses don't naturally float like ducks do and if you get them out in the water they're too busy swimming to attack you.