As mentioned by Stewart_Reptiles, it's a personal choice and is also dependent on where you live. I use aspen because their natural air temperature is humid, and not the ground, but that's just a personal preference. I also find that it's one of the more cheaper types of substrate. As long as you're happy with the substrate, have proper humidity, and a healthy bp, then it's all good.
Side question: Anyone try mixing aspen with any other substrate? I hear of different substrate mixes but none with aspen so far. Just looking for something that might look nicer or provide more benefits.