Yeah, it basically depends on what you want to keep I think. For "reef," the front-to-back dimension is usually the most limiting factor when designing rockwork. When you get into those larger breadths like that, it opens up more possibilities for ravines, interesting crevasses, etc.
But then again, a 5' tank gives you that extra foot of swimming space so the fish might appreciate that more.
I dunno ... it's hard to say.
...
I know! If both tanks will fit .... How about a tank that is 60x24x24? That's only ... um .... 150 gallons?
