The Express has a plastic trigger guard, the Wingmaster and Police have aluminum.
Other than the quality of finish that's about the only really major differences in the three.
All three are made from the same forged and milled receiver, and the parts are pretty much the same.
The major difference is, the Wingmaster has the nice walnut stocks, polished blue, and appearance goodies.
The Police has a tougher parkerized finish, and a couple of beefed-up springs.
The Express has "hard wood" stocks, a rough black finish, and the above mentioned plastic trigger guard. Plus they're a little rougher and less well finished inside and out, including the bore.
Bottom line is, other than the quality of finish, you're getting mechanically the same gun. They don't use cheaper, or weaker parts.