My sand boa will not eat feeders unless in a separate deli-cup. He will burrow and not even look at them.
I like the walnut, because it has much more burrowing capability. Since my boa will only eat every 2 months or so (Even though I try every week) he doesn't digest the walnut because he's in a separate deli cup.
Also, the walnut will only stick to feeders if wet.

Also, walnut is like sand, but without the dust and messy clean-ups.