Unless you're buying tanks to keep a specific type of fish that do better in tall tanks (angels or discuss for example) ALWAYS go with longs! I even preffer 40 and 33 gallon tanks over 55's for the common 48x12" layouts. And if you decide one day you'd like to graduate to larger tanks, if you have the space besides long, I always suggest wide as well! I tend to not look at pure gallons, but foot prints. A 55 has 15 more gallons then my 30 breeders, but the 30 at 36x18" has a better foot print for territories.