what are you thinking about ...for in the tank...as stated above ,,,MOST spieces would fair well in a 40 gallon breeder...but if you want a community for instance the 55 gives the fish more space to seperate from each other both vertically and is a foot longer.. but for a single fish...for instance a dempsey ,,,,the fish might like either......
depends, for fish that don't exeed 7-8 inches, a 55 is better... 40 breeder is for fish that are about 8-10 inches in size... of corse community fish will be better in a 55 beacause they won't need the with... but a fish like a green terror or jack dempsey would aprestate the 40 breeder more...
I prefer the 40 over the 55 for the type of fish that I personally keep, I really dig the extra depth that allows for slightly larger fish. Not to mention the ease of maintnance, with no center brace it's a snap to clean.
As others have already mentioned the 55 does have advantages when it comes to taller fish and with smaller community setups.