I think that you can design your levels faster with
WED, until you are good with modeling.... if you are good
at modeling, you can use MED or another modeler just as well....
so if you want to get things designed fast, it could be a good idea
to use WED for a while...

Also, GED comes with the new beta of Gamestudio.
do a forum search for "GED", and there is a download link....
once you use GED, you might not want to use WED, though.
GED is awesome and has some neat terrain editing too
(and GED is realtime, you can see shaders, too!)

-jim