Well I have no experience using world composition but from what I understand is helpful in the construction of very large maps. Level streaming on the other hand has the misfortune of being named level streaming as to function when in fact functions more along the line of 3ds Max’s x-refercing system A level can be set to stream based on a blueprint or set to be loaded at all times A level can contain master lighting components or even set up as a lighting scenario to switch between day and night lighting. Verminous elements based on game types can bee added so that a singe map can be used for any game type configuration.
Streaming levels can also be used in team based environment development as with the x-referencing 100’s of levels can be worked on by 100’s of individuals.
Personally I’ve been using level streaming much the same way as I do in 3ds Max using x-reference as it is faster, more control over the environment and allows for a much better workflow pipeline between the two apps.