I was going to ask their age, but assumed they were the same age. Thank you for clarifying.

As far as the substrate goes, that "plain" soil contains fertilizer (the little white balls or rocks). Eco-earth is very cheap, I've never experienced it molding. I change substrate every 1-2 months, which most will recommend. Sometimes cheaper is good for you, but not for your animals.

I believe a few threads down, there is a question regarding substrate, I hope that thread helps you.