Bonsai Nut
Nuttier than your average Nut
WIDE is better than multi-level. When you go multi-level, your trees on the lower levels back branches will get screwed up as they are either shoved up against the support behind them or shaded out. This problem increases with the size of your trees--If you have bigger trees, you will get broken or weak back branches.
I've never added levels to any of my stands, only increased their widths--wide is better...
I wouldn't say one is "better" than the other - like so many things in bonsai, it depends on your situation. If you are placing the shelving up against a wall or a fence (or in my case tall bushes), having a 36" wide shelf with two rows of tree on it is difficult to work with - while the same setup may be perfect if you have an open space in a yard or field. Place a tiered shelf facing the north, and the bottom shelf will get shaded out. Place a tiered shelf facing south, and the bottom shelf gets the same amount of sun as the top.
I find that having tiered shelving works for me in some cases, but not in others. I like the ability to have shelves where I can put deciduous on the lower shelf that won't get as much sun as the conifers I put on the top shelf. But hey, to each their own. What works for me isn't going to work for everyone... and I guarantee you the inverse is true.
I almost didn't post because this is turning into a soil wars thread In a perfect world, I'd have each of my trees on its own dedicated stand... with a turn table