Length or Height more important for tank?

[Pan]

Just trying to figure out an idea for a tank I am looking at constructing. I was just wondering peoples opinions on height vs length. I know width is an important aspect for certain things, mostly imo aquascaping, and for certain larger fish they need the turning radius, but I'm looking at doing a long pennisula style tank about 18-20 inches high...have not decide on length but fairly long, and most likel 18-20 inches wide...I have a drawing a friend did and if it work i think my vision would look quite nice. I know Tangs would like the long swimming room, but do they like height, what about other fish. I realized I've never looked into that aspect of fish before, always had a 3 foot deep tankuntil lately. I know the school of chromis will be happy as well as a few other fish..
Still in the planning stages. It would be viewable from both sides and one end.