Gravel/sand is like many things a matter of preference. I grow a lot of plants and use sand. I use greywacke river sand because there is plenty of it here cheap and it is smooth and relatively inert. Some plants like a bit of calcium so some beach sands can be smooth and provide that. The specialty plant tubes have a number of peaks (3 on blue I think) and as you say these are the important wavelengths. They have different functions, some encourage flowering, compact growth etc. We are geared to greern but plants are not so as with many things it is about what you want to achieve and personal preference. Some will be easier on the eye but not so good for plant growth. Often a combination is a good compromise.