Hi NatureFish,
When I set up my new 4'x2'x2' planted I decided to make my own substrate. Before reaching that decision I did investigate using Eco Complete and ADA Aquasoil but decided against them on cost. Top soil or potting mix is "dirt" cheap (pardon the pun). However, I wouldn't recommend this approach unless you know what you're doing as it can go horridly wrong. I suggest first reading Dianna Walstad's Ecology of the planted Aquarium before deciding to go down this path.
The key to successfully growing aquatic plants is getting the right balance of CO2, nutrient and light. Having a soil substrate helps with the first two.
If you are interested I've posted a picture of the tank set up and how I prepared the substrate here...
http://www.aquariumadvice.com/forums/f24/dirting-an-established-tank-question-335455.html
Having used potting mix last time I will be setting up another tank soon with ordinary garden soil to compare the difference.
Anyway good luck with the new tank what ever you decide.