I vote aspen, I have used coconut fiber before and found that one of my snakes was a burrower and often had it packed in its nostrils which concerned me, did not want to have it cause a respiratory infection. I then switched to aspen and it is far easier to clean, and soooo much cheaper.
You can easily see exactly where the snake has done their business and it dries up in to a nice neat clump of aspen shavings, very easy to remove. This then reduces the frequency of having to completely change out the substrate in the tank. As well I and get 8 cubic feet of shaved aspen for $15 and I had to pay the same amount for enough coconut fiber to do two 30 gallon long tanks once! The big bale of aspen I have had since May and have not even used 1/2 of it with 5 snakes in the house.