I think this whole thing about fish enjoying length is just because most tanks don't have any width and rock occupies the entire backside...so, of course, the length becomes pretty important!
I think the best dimensions for the fish would be a square....as large as possible.
Having said that, I don't think that 6" width is worth the trade off of 2 feet of extra length. In other words, 24"W or 30"W...neither is wide enough so you might as well take the 96"L.