I believe I read years ago that some of the internal parts on the express are stamped rather than forged/machined like the wingmaster.
Also, the finish on the wingmaster is a highly polished blue whereas the express is a matte finish.
I've been using a express super magnum for years without any trouble in the muddy waterfowl fields. The only 'trouble' I had was extraction problems on extremely cold days which was my fault and caused by a gummed up extractor. I took it apart and cleaned it up - now no worries.