I guess it would be client, I am doing a single player game, no server, no networking. World Composition is for large worlds where it automatically controls the streaming in and out of sub levels (cells) as you walk around (and controls distant LOD). Where as level streaming has to be specially setup with either distance based volumes, or trigger volumes.
My game so far has twenty distinct worlds, and each have numerous interior streaming levels (So I am using a combination of World Composition for the exterior, and leaving streaming for the interior).