I really don't see much if any yield increase between April and May beans....it all depends on the year.    Got rain?    The advantage for me is to shoot for an early harvest by the end of Sept, first of October.    Around here, one starts to get October rains, humid,  cloudy, and shorter days as the fall goes on.  If the beans get a good soaking say mid to late October, one will play holy heck hoping for them to dry down again.   I've seen guys cutting beans in December for that very reason.