Flourite & Eco-Complete are both good substrates but IMO Eco-Complete beats Flourite hands down. Flourite needs to be cleaned before use. Once you think you have cleaned the Flourite enough, clean it some more. Eco-Complete is just open the bag & pour it in, no rinsing at all. Eco-Complete is half the price of Flourite, at least here in Aus it is. Regular Flourite has rough granules that can be hard on the barbels of fish like Cories & Loaches. Eco-Complete is much finer. Flourite sand is good for Corys etc but it doesn not have the same amount of water flow through it as the Eco-Complete does. Performance wise they are virtually identical once you have them in the tank. I've used both & I can safely say that I will never use Flourite ever again.
ADA Aqua-soils are really good but they cost. The ADA soils will supply nutrients for a very long time & not really need to be topped up with substrate tabs whereas both Eco-Complete & Flourite will need to be topped up with the regular use of nutrient tabs after about the first 6 months of thier life.
Gravel can work, but is a long way from being the best plant substrate.
How much substrate material you need depends on what the floor dimensions of the tank are & how much hard decor like rocks & logs will take up space in the floor of the tank. Hard decor will displace some of the substrate material, meaning you will need less of it. Assuming a 48x24" footprint you would probably need somewhere around 5-8 x 20lb bags of Eco-Complete & the same of Flourite.
This isn't directed at you Jeta, so please don't take this the wrong way. This is directed at substrate manufacturers that like to make up these ridiculous 'rules'.
These 'X amount of substrate per galon' rules are the stupidest thing I have ever heard of. They are worse than any WPG or inch per gallon rule. Two tanks can have the same volume but have a very different footprint. You could have a tank that is 72x18x22" with a volume of 124gal. The footprint of that tank is 1296" square. You could also have another tank that has dimensions of 48x24x30" also with a volume of 124gal. The footprint of that tank is only 960" square. Same volume tank but dramatically different floor sizes. Clearly the tank with the smaller floor size will require much less substrate to achieve a given depth than the larger tank would.
I'd say 8-10 x 20lb bags of either Eco-Complete or Flourite should give you 2.5"-3" depth if you lay it out evenly. Having the front centre shallower than the back & sides like Funkman described is the best way to lay the substrate.