I'm a newbie to Iwagumi too, just received some very nice pieces of petrified rock the other day for my own little project, though it'll be a few months before I really begin. The comments below are simply my perception of the style after having done some reading about it.
In researching the style, it seems the most important overall aspect is the use of the golden ratio/section. That's why, to many including myself, the ADA 60-P aquariums are so attractive, it is almost perfectly in proportion horizontally to vertically. Following that ratio for placement of the rocks will also add almost instant beauty points.
With Iwagumi the rocks themselves really are the most important factor inside of the tank, poorly chosen rocks will definitely detract from the beauty points.
If the rocks aren't in proportion to the tank it's not going to look quite right, you'll find that a lot when others are saying 'the rocks are too small' in the Photo Album, but you'll also see examples of the rocks being far too large and overpowering.
In answer to which aquarium would be better, as you might have guessed by now, I say whichever one is most 'correctly' proportionate in regards to the golden ratio, provided the height isn't so great as to be a detriment.
I hope that helps.