It's a hard choice: if only one is reef ready go with that. But tangs swim up and down as well as longwise, and among other fish you get more jumpers in a shallower tank. I'd say to support tangs well, get some strong flow in there, and a good skimmer and maybe a fuge for good oxygenation. Which is a way of saying the stand might make a difference too, in what you had room to do under the stand.