I think that the cube tanks allow for a more natural aquascape and are just more pleasing visually. I always wanted the 20L but once I got it and set it up I was not satisfied. I just personally love the shallow cube tanks, at least 24" square and no more than about 18" tal with a pendant-style light fixture so you can look in from above. Very cool look to me. It is ALL a matter of preference though. I just think that depth front to back is the most important part of a reef tank. I wouldnt buy another tank that isnt at least 15"-18" front to back and IMO the tank shouldnt be taller than it is deep (front to back that is). For example, my fav tank sizes are the 120g (48"v24"x24") and the 40g square (24"x24"x16"). I must say that one of the worst sized tank for a reef setup is the 55g because the 48" length and 21" height but only 12" front to back... REALLY tough to properly aquascape that tank.
Jesse
BTW, SaltWaterNewb, doubletail, and Ritsuko, :welcome to nanotank.com!