There is no difference except price. While the prop is spinning and you hit bottom there won't be any difference. Heavy duty props might survive a big hit better but in the long run they are the same. Save some $$ have you old prop re-conditioned.
I don't think they are talking about the strength of the prop, they are talking about the size of the BOAT, light duty is for a small aluminum boat or such, heavy duty is for a kicker on a bigger boat.