I was where you are about a month-and-a-half ago and after all my research and asking people's opinions I decided to go with straight eco-complete (black). It looks fantastic in my 30 gal and is a great contrast to my green, green plants
Be aware that if you are looking at darker cories they will tend to blend in with the substrate. I added root tabs about 2 weeks ago and my plants have seemed to really of taken off. CO2 is next in my plans! I vote eco just because it looks awesome and doesn't cloud your tank. I've been thinking about adding some black sand in the back of the tank just for my cories...I'm sure they'd love it.