Between the aspen and oak, I'd go with oak. Aspen is too fuzzy and has little or no character. There are probably prettier choices than either of those two. Hard maple is hard to beat. If you are near a Rockler Woodworking store, they often have small pieces of exotic woods that would dress up the riser. A darker wood would create a nice contrast with the hickory.
I've tried to work with aspen and now avoid it like poison! Poplar would be a much better choice if you are looking for inexpensive woods. The sap wood is green, which may or may not be to your liking. The heart wood is similar in color to aspen, but has more grain. It also doesn't fuzz like aspen.