Which fish first

[gregzz4]

Definately go with a depth you are comfortable with or you'll be kicking yourself later
With 3' width you're going to be on a step ladder anyway (to reach the back) so going a bit on the tall side may not matter
I'd be thinking about light penetration, coral placement and LR structures' sizes if I ever planned a tank this size

Maybe someone like Bblinks or The Grizz could give you some insight into dealing with a tank with those dimensions