Aspen shavings are best for everyday use, safe, unlikely to impact the snake, best simulation of a forest floor allowing for burrowing, ideal characteristics for maintain proper humidity and sanitation. Plain white, no-chemical, no-scent paper towels are best for a "hospital tank" if you need to accurately observe the timing of defecation or regurgitation if you think you're snake is ill, or if your snake is new and you're getting to know it's cycles. Granted that they aren't as attractive as aspen chips, but if absolute safety and observability of excretions are paramount, then paper towels are the way to go. Nearly everything else has substantial disadvantages.
Never feed your snake on anything with loose particles, even aspen. The particles will stick to the food and get swallowed, possibly causing impaction. Always feed on plain white paper towel in a separate plastic box.
Aspen is non-toxic unlike other woods like pine and cedar, and is least likely to cause impaction from a snake merely living in it without feeding on it.
SAND, INCLUDING CALCI-SAND MADE FOR DESERT SNAKES LIKE RATTLESNAKES, SHOULD BE AVOIDED LIKE THE PLAGUE. EVEN IF YOU DON'T FEED ON THE SAND, IT WILL GET IN THE MOUTH AND CLOACA AND CAUSE PROBLEMS WITH IMPACTION, NOT TO MENTION ITS DEHYDRATING AND ABRASIVE EFFECTS. USING CALCI-SAND IS A COMMON MISTAKE FOR NEW SNAKE OWNERS.
As-pen whatever I need to keep my snakes healthy and happy,
Doctor Mike