i really hate the 55's. I loved the lenght of mine, but they're so narrow that it is really really hard to get good rockwork in there. You almost cant realy build upwards (because you need bigger pieces on the bottom which are just too wide for the tank to look good with) and you pretty much have to build your rock low and lenghtwise.
I've got a 40 breeder now. it is only 36 inches long, but its like 17 inches front to back. I love how easy it is to build in there. I personally like the look of lots of open sand in the front, along with lots of open swimming room. But i also like building upwards with the rock. It adds height to the tank. But with 55's, you lose the open swimming space and open sand when you go upwards (again because you need larger/wider rocks on the bottom..they take up all the room from front to back). So theres not a whole lot of options with it.
Now with my 40 breeder...i can build as high as i want, but still have plenty of open space. Overall it just looks much nicer imo
. Some people can pull off the good rockwork on a 55..but its hard and if you dont get the "right" rock, then its even harder.
But either way...theres not too many things that you'd be able to keep in a 55 gallon tank that you COULDNT keep in a 50. I cant think of anything offhand to be honest.