Seems quite simple! Those that fly a level segment set bugs to MDA, those that fly a constant descent set a minimum DECISION height of MDA +50-100 so as not to infringe MDA. The reason some companies in the UK went to constant descent was that during the level segment to the MAP some pilots were getting visual contact with the runway(way too late) and performing a dirty dive to get in. IHMO I don't particularly like flying level at 300 feet fully configured.