I heared that long is best for filtration, but I dont know if that is true or not (anyone?) Ive also heared its harder to plant tall tanks, so if your gonna want to do that I'd think about the long. Like GuppyGuy said it depends on the fish you plan to get, if you want a lot of bottom feeders, then your gonna need the long, but if you want an angle fish then a tall would be more apropriate.