I like the 54x54x24. Although at 300 gal, the dimensions that invincible569's famous tank (now owned by agentSPS) had were very cool. That said, 5' or less is much easier to light IMO. I also might throw out that I'm kinda against the current trend on this, but for aesthetics I tend to like taller tanks, just don't like not being able to reach the bottom. Depending on your arm length you might consider if you can go 25 or 26" deep -- I went 26" deep on my tank and I think its perfect, I can reach the bottom to work (unlike on my old 30" tank), but there is a ton of vertical room for fish and coral growth.