MIneralized topsoil is never a consistency other than "mud". If it is a gravel-like consistency it isn't MTS. It has to be covered with an inch or more of a more gravel-like substrate, but the only advantage of one substrate over another would be the CEC. Flourite has a high CEC, and I think Eco Complete does too, but possibly not as high. Then, there is personal preference, for what the substrate looks like - some people like the appearance of one more than another.
You can use a heating cable with any substrate, but it won't provide any worthwhile benefit with any substrate. I doubt it doing any harm either.