After the original wood rotting out, my city replaced the slats in the public park benches with ipe. Hard, dense, and heavy, it's stood up very well to the weather, salt spray from the snowplows, direct sun, etc., turning to a nice even gray color. It's going to outlast the cast aluminum frames. If you build a light construction, I think they will not get too heavy. Teak is another good choice, but quite expensive.

I'd also recommend doing a mock up / prototype in cheaper wood prior to starting in on your project. Stool samples, if you will. :-)

Here is a link that might be useful: stool sample

This post was edited by bobsmyuncle on Sat, Jan 26, 13 at 18:45