It's all about surface area (assuming its reasonably tall) - they don't really use the height so extra height will not really allow you to stock any more fish. Better off getting the longest tank you can fit in.
I was looking at a 60 gallon which was 48 inches long, but only 13 inches wide. Would a 40 breeder be just as good because it is wider and shorter? Or is length more important than width?
You want mainly length and depth (front to back) assuming you have Mbuna, if you have peacocks, you will want te height. Mbuna and rock dwellers so will generally use as muh fo the tank as you have rock in, peacocks are more open water..