Aspen is fine, won't hurt them. Kiln dried pine is fine too. The little bit of product that would make it to your snakes on normal feeders (ie, not wet), shouldn't cause issue.

I really liked the pine pellets and the way they smelled nice and fresh even after a couple days. The issues for me were that they were hard to get hold of at times (didn't have much space to store extra), and that everything would end up coated in a layer of dust from the pellets rather quickly. You can put a layer of wood shavings on top to minimize it, but it still can get dusty.

Of course, it may not have helped that I use serious fan-power to move air around down there and keep things circulating. But next year when the snakes/rats will have their own buildings, I'll be using them again definitely. At least in potty corners. For now I'm just using kiln dried pine bought in bales at the local feed store.