if you look on SRS you should find pics of the differences. The manifolds have had many supercessions in the parts catalogue, and the early ones had a habit of falling apart internally. Most of the faulty ones will have been sorted under warrnaty etc by this point, and it seems unlikely you'll find a faulty set still in use. Ofcourse if you have the turbos off for some reason, it pays to check what you have against the photos to ensure they are the revised type.
Later manifolds were improved internally to stop them disintegrating. Theres no real performance advantage, and the later S4's and RS4 all used the same manifolds.