So... first off, you should not rely on your substrate to change your water parameters. Your substrate will stop changing your water parameters most likely after a month or two. Your tank (assuming its populated by plants/fish etc) will still have lower ph but that's true for any populated aquarium. Mum, fish poop, etc will lower ph slightly all by itself.
Secondly, eco-complete is inert. Meaning it does not have any nutrients necessary to keep plants alive. It does have a higher cec value then regular gravel but that by itself doesn't mean much.
Brightwell on the other hand is an aquasoil, meaning its essentially baked dirt and thus contains quite a bit of nutrients. You will still need to fertilize your tank to get the best out of your plants in the medium and long term, but it does provide nutrients to them right away.
A lot of folks on this forum (myself included) really hate eco-complete. A few like it quite a bit. My personal complaint with it is that it doesn't look very good, and its difficult to plant into.
So what does this mean for your choices?
I would say what you pick should be determined by what aesthetically you like. If you think eco-complete looks better then brightwell, then you should "cap" the brightwell with the eco-complete. If you don't much like the eco-complete, then just use the brightwell.
A third option would be to use the brightwell and cap it with pool filter sand if you like the aesthetics of a sand tank more.