It depends on the site of the course. If the course is in such an area that will allow for natural, scruffy looking bunkers, then just keep them like that. In that case there is no need to create bunkers that look artifical. However, when presented with a site that will not allow for natural bunkering, then the design that requires the least maintenance is best. In many cases, that is the flat bottomed, grass faced bunkers. The grass will hold the face in place, resisting erosion, and the sand in the bottom will not be subject to erosion either. It could however lead to standing water in the bonkers, depending on the subsoil conditions. However, for greenside conditions, I think that steep faced, flat bottom pot bunkers surrounded by Run-off areas are the least labor intensive of all options.