As far as substrate, I have never used Flourite or Eco Complete, so I can't comment on them. I have used Amazonia, and was very impressed with the results. It has lots of nutrients to give plants a real jump start. Those nutrients get depleted, so you will need to start dosing the water column before too long. But even depleted Amazonia grows plants well as long as you dose appropriately. It is pricey, though. I've only ever used it in nano tanks, and don't even want to think about how much it would cost for a 75 gallon.

One of my tanks has the Mineralized Topsoil Substrate that Jim mentioned. Time consuming to mineralize the soil, but it is very nice not having to dose the tank!
My other tanks have fired clay substrates like Soilmaster Select, and Schultz Aquatic Plant Soil. Soilmaster isn't available anymore (and I don't know about the Schultz), but a comparable product is Turface. They have the advantage of being very cheap, and plants root in them easily. They have no nutrients, but are able to grab nutrients from the water column and make them available to plant roots. They are very lightweight, and it can be hard to keep plants down until they root. It's never bothered me very much, but some people find it a pain.