For a 150, that's a heck of a lot of substrate. Unless you have a couple hundred bucks to spend on it, I'd suggest one of the cheaper alternatives. Shoot for 3" total. You can use pool filter sand or Turface, both of which are very cheap.
Or, you can use dirt, Miracle Gro Organic Potting Mix, or mineralized topsoil for the bottom layer and cap it with whatever you want.
I don't like layering - it tends to make a mess of my tanks whenever I pull out really rooty plants. Turface is sold at John Deere Agricultural stores, and is basically the same thing as Flourite - baked clay. High nutrient absorption rate (CEC), so just plug in some root tabs and you're good to go.
And, remember, whatever you pick, rinse the heck out of it first.