See added notes in my previous post about original documents at the DNR.
IMO a well designed spillway will handle and discharge a lot more water volume compared to a overflow pipe. However the overflow pipe allows slow release vs rapid release of a spillway. One has to consider the volume capability of the receiving stream.
Mike Otto of Pond Boss magazine wrote an article in the magazine about spillways and their design. Let me see if I can find that article in a back issue.
