It seems to me that you don't mind the extra costs, but how about this...what color do you like? With you're red flint, laterite, and possible flourite mixture, you'll be getting a reddish substrate look. With eco-complete you'll be getting a black look. Maybe that could help you finalize your decisions. As far as what's better or not, I do think eco-complete would provide better results, but I think the difference is so small that it's not worth giving up your current substrate for. As some people have reported, plants mainly get their nutrients from the water column. If you have a good nutrient dosage routine, then your plants will do fine. Some people even said that they can grow things in plain gravel. Plus, you have laterite, and I'm positive that is going to help. If you're plants decide to look for nutrients from the substrate, the laterite will provide whatever it needs.
You don't have to give up you're current plans unless ... you want a totally different look?
You don't have to give up you're current plans unless ... you want a totally different look?