I have a bunch of pot scrubbers, matrix, some fluval bio rings, and some filter floss in my sump and my matrix appears to have more growth on them than any of the other medias, aside from the filter floss at the top.
With that being said, I'm sure there's a thin film of BB on the pot scrubbers, just not enough to gunk up like the matrix (the matrix has a sludge/film on the surface).
So, I think the pot scrubbers stay cleaner, but don't have as much room as the matrix. But then again, that's my tank setup: all of my media is submerged and I'm sure everyone will have somewhat different results.
If you wanted to get some pot scrubbers, let me know: I've got a box of those things sitting out in the garage and I'd let them go almost free.
Edit: I see that someone else mentioned the K1 media (they also make K3). I have a moving bed of the K1 in the discus tank and it seems to be doing okay. I don't have any complaints from it other than they tend to snag on my filter socks, but the sump design on that tank is only temporary.