Aspen can be found in wood chips and shavings which is more ideal than the thin strip kind, but usually costs more. It costs more than cedar and pine and is safer than both for guinea pigs, but not ideal b/c it does contain some dust although a better option than pine. I find the most economical solution is using fleece. I buy pet rugs which are fleece rugs that can be cut to fit your cage and washed and reused. It works great b/c you don't have to dig through all the bedding to clean out all the poo. The pee goes right through the fleece, so you must put something beneath to absorb it. That is what I use the wood shavings/chips for. I use pine. I put down a layer of newspapers, then pine, and then a layer of paper towels to help keep the wood chips from sticking to the fleece. Still avoid cedar the smell may be too much. Aspen can be used the same way, but costs more than pine.