I am not familar with aspen. Are you sure it isn't Poplar? Aspen in a very soft wood (see here
http://www.extension.purdue.edu/extm.../FNR-273-W.pdf). Poplar is a good wood for heavy stained or painted trim in buildings. It tends to sometimes have a greenish tint which affects lighter stains. I have used it in situations were the client wanted a cherry, mahogony or walnut look but didn't want to pay for the wood. It is tight grained and stable. I've never used it outdoors so do not know how it will react to moisture (warpage, expansion, etc.). Buy a piece and play with it with different finishes and water/heat torture.