I'd agree that multiple pieces are more versatile than 1 huge piece.
But what you do with it really depends upon what you're trying to achieve. I like to use wood to add some vertical interest to what might otherwise be a pretty flat-looking tank.
Also, keep in mind that the wood will leak tanins into the water. Mopani can do this a lot. This is great if that's what you're going for. But if it's not, one standard way of helping to reduce it is to boil your wood (like making tea) a few times until the water doesn't turn as dark during each boil. If you get one huge piece of wood, you're not going to be able to fit it in anything besides your bathtub for a soak.
Boiling is also good to kill anything that might be on the surface of the wood. I'd also give it a scrub with a stiff brush. Mopani is super hard, so I've actually used a wire brush to help accentuate the great textures. The nooks and crannies you can't get to stay darker, and the exposed areas become highlighted. Just be prepared to use some elbow grease.
As with rocks, make sure that any construction you do with wood isn't going to collapse if some inhabitant moves the substrate around some.
Also, mopani has all sorts of texture. It's a perfect place to anchor different types of fern, moss, anubias, or to make little wooden caves.