40b all day!! IMO they have one of the best footprints for reef tanks, and will be so much easier to work in and light. 55s make odd looking reefs IMHO to talk and not enough depth. Here's my pride and joy 40b
40B. Better depth for aquascaping and more surface area for gas exchange. Some argue that a 40b can be stocked as heavily as a 55 because of the better gas exchange. If youruip in the air, get a 65 or 70.