not if you think of it height wise, but the 40 gallon has more floor space. The 55 gallon has 4 square feet, but a 40 breeder has 4 1/2 square feet.
Either can be used. If you have a very large beardie you would be better off with something larger, or at least the 40 breeder (because it can be hard for them to turn in the narrow tank). Right now I have a tank that's 36" long, 18" deep, and 25" tall for my beardie. The problem with tall tanks is that my beardie doesn't like basking up high all the time, so he usually doesn't get close enough to the UV bulbs. I supplement his food, and a lot of breeders raise beautiful beardeds without any UVB lighting, but I would like it if he got closer to it.
So if you get a 40 gallon, a 55 would be a pointless upgrade (unless the beardie likes to climb and is 18" or less in length).
-ryan