rabbets make a stronger hive than butt joints, and there's less end grain exposed to the elements so they'll last longer, too. to be honest, butt joints work just fine for these hives, but if you plan to abuse the boxes at all, or if you just want a really sturdy hive, rabbets are probably the way to go. I like to use double rabbets, which are substantially stronger than the more common and boring single rabbets. uses a little more wood, and takes a bit more time, so it's a few bucks more. thirty-five bucks more.
rabbet joint on a Warré roof