I would always choose the 40 breeder. It holds more than 40 gallons while the 55 holds less than 55 gallons. I also like the footprint of the 40 breeder better. It also suits my choice of fish better, who like longer and wider more RIVER type tanks. But I guess it's all down to personal preference. I have a 55 gallon but I got it when I first started the hobby many years ago.