There are a continuum of healthy and not-so-healthy choices for substrate. It could be taken to an extreme (rediculous example alert) of just keeping them on glass and bleaching it daily. At the other end is probably fecally-covered soil. Someone will always say that one substrate is better than another, and most of the time they'll be correct, but unless you want to end up bleaching your glass substrate daily, most of the choices in between are probably just fine.

I've never used aspen bedding, but it's attractive, has anti-microbial properties, and is fairly cheap. I think the main concern is that it sticks to moist foods and can be easily ingested. Perhaps someone who has used it for quite a while could offer better advice. If you choose, it, you might decide to feed them in another enclosure or offer their food in a bowl to minimize the amount of aspen that they might ingest.

Crazyhedgehog did a great job of discussing the pros and cons of various substrates in the Care Sheet Section. Have a look: (http://www.thamnophis.com/caresheets...ake_Care_Sheet).