I won't dignify my little project by calling it an experiment as there are too many variables; however, the results so far are interesting to me. I set up one small tank with a sand substrate and DIY CO2, no regulator,etc. and one small tank with potting soil and sand substrate and no CO2. Both tanks have received 1 weekly dose of Flourish Comp and a 50% weekly water change. The CO2 tank housed 1 betta and some snails and the soil tank just had snails. The lighting is "roughly" the same. Both tanks have produced very healthy root systems. Even the stem plants that were planted in the substrate developed lush roots. The CO2 tank seemed to produce bigger, taller plants. The soil tank produced more compact plants with more leafs.
One thing that I have found interesting is that some plants in the soil tank actually drooped over night and stood back up after the light had been on for a while.
Anyway, hope this has been of use and I would appreciate any insight / info on the results.