Is there a reason you don't want to go for a 30 (36 X 12 X 18 for height)? I've got some 15s that are only 12 inches tall and that's not much height.
You'd asked about light in an earlier post. I've got twin tube fluorescent on one 30 gallon--20 watts per foot (no CO2 on this one) and a power compact on the other--32.5 watts/ft (I like watts/ft better than watt/gallon). The pc light is a tad much for my diy CO2, but I just use a few floating plants to reduce the intensity. If you go with the shorter tank, then I'd go for the fluorescent lights instead of the pc. I've got pc lights on my 15s and it took quite a while for them to get happy with that much light (higher CO2 is needed).