I find that bedding, like the aspen you were thinking of using, tends to fly out of the cage more, along with the poop. You will have to clean it out at least once a week, and it can be expensive.
Fleece doesn't let the poop fly out, except for the occasional piece or two. I used towels under the fleece but it has to be cleaned WAY more often because the towels soak up a lot of pee. At a lot of feed stores they sell pellets you can put under the fleece or use as bedding. But there will probably be a build up of hair in your washer and/or dryer, so you will need to find a way to get the hair off.
Someone I know uses it for her guinea pigs and it doesn't have to be cleaned as often, if under fleece only once a month or more. If you used them as bedding, you will probably have to clean it out more often, every two weeks or less. The feed stores sell big bags for around $5-10, and they last a long time.