Just started using the Pond MAtrix. I see what you mean about pebbles different colours etc. But they are so light.
I know people are using lava rock, scrubbies, eheim etc, but I came across a neat idea on another forum using bonsai pummice stone. you can get a 5lb box in the USA for about $20. lots of surface area for beneficial bacteria to grow.
Another thing I was contemplating was whether or not there could be too much media in the filters, specifically to a planted tank as it may become too effective in removing thing like nitrates -just a thought