If I remember correctly, the statement made about aluminum vs. steel was in reference to the Dahon slipping seatpost problem. The poster contended that steel was better because the seatpost doesn't slip on the poster's Brompton (which is steel). There was no basis in fact provided.

Generally, steel offers a "softer" ride than aluminum which is stiffer than steel but the trade off is a harsher ride b/c aluminum is less flexible than steel. I own bikes made of steel, aluminum and carbon fiber. There are differences but for a folder, I didn't consider the material. I looked at the fold and the component spec. Though many Dahon's like my Curve D3 are spec'd with Big Apples which are plenty plush and pretty negates any harshness attributable to the aluminum frame.