I agree with reptileguy the smoother the better, but if I had to pick between the 2, and this is my personal preference I'd go with lava rock rather than slate (no personal experience with it, I use river/cobble stones in mine, but from what I've seen alot of malawi tanks have lava piles in them and fish seem to be doing fine), for 2 reasons, there's a greater surface for algae that mbuna will eventually be eating and you can make bigger caves with larger rounder rocks, you can also stack them as high as you want, pretty much to the surface if you have enough lava rock.