24" front to back will be a lot nicer than 18" front to back, but is it enough to decide to go 48" over 72"?
Another consideration, lighting.
With a 48"X24" tank, lighting will be much less expensive. You can light it with 2X250MH or 8X54watt T5HO.
A 72" tank will require 3XMH bulbs and either if you decide to go with T5HO, you'll need to probably go with 16X39 watt (3') or staggered 80 watt (5').
If you go with a fixture, a 6' fixture will be MUCH more expensive than a 4' fixture. 4' fixtures fit a wide variety of tanks, so more are made and they're considerably less expensive. If you go retrofit, you'll cut down a lot on the large difference in pricing, between 4' and 6'.
Also, take into consideration how much room you have, in your home, which I'm sure you've already thought about.
I think my main deciding factor would be the cost difference, in lighting. Personally, I'd rather have the 24" front to back, than the 6' length.
If it were completely up to me, and budget wasn't a factor, I'd go with the 300 Deep Dimension, 6'X3'X27" tall! My 200DD was HUGE when I set it up. It is already too small...lol