I tried both DIY co2 and excel, separately, and together.. the DIY co2 alone yielded the best results in my tank.
and when i say best results, i mean vals that grew 2-4 inches per DAY, pennywort that grew an inch every 2 days, moneywort that took over the top of the tank like anacharis, etc etc.. my tank, with DIY co2, went from moderately planted to jungle in less than a month.