It depends alot on your plants. Some plants are major water column feeders. In this case I would think normal gravel should be fine because they get their nutrients from the water. Others are heavy substrate feeders - this is where my thoughts below might help. Be careful too, some plants just attach to objects like java fern and most anubias (I think). This might be an option to get you plants without pots - just and idea.
I like any of the Seachem planted substrates and come in brown, redder brown and black. I like the simple Seachem Flourite (fiance has the red version but it seems too red to me) but I am considering the Black Onyx line as well. Looks nice and comes in a sand version.
I do not have access to Schultz Aquatic soil - hard to find in my area. So I not comment on this - yet many people seem to like it.
Eco-Complete is a little more expensive but I have too heard rave reviews on it. From some peoples experience it can affect your PH, but since you will use so little of it, I dont know if it will matter.
Dont only consider commercially available options either. I dont utilize this method but some use peat, soils and gravel mixtures which are usually cheaper than the commercially availble options. Do quote me on this because I have no knowledge on it, I am just aware of it. You will have to do more research on how to do this properly.
Good luck
Aries