Assuming you are talking about a new standard model Magnum in 12 Gauge...
The Wingmaster is polished blue, a tad lighter on the barrel end, and quicker, with a metal trigger group, a gloss walnut stock, twin beads, and a fancier magazine cap retainer.
The Express has a rough finish, a plastic trigger group and nut retainer, one bead and a matte-finished laminate wood stock. I think mine is a tad more sluggish than an equivalent Wingmaster, but Remington has changed the barrel so I can't say if that's still true. Also, "sluggish" can be good for longer ranges and trap, but bad for close flushing birds and skeet. Depends what you want. You can always go with a 26" barrel, which makes the gun quicker anyway.
Over the years, the Express has had different wood (current laminate is nicest, IMO, and worth a few extra bucks), different ribs (again, the current model has the nicest rib). Wingmasters have also risen from the everyday 870 to the upscale field model, since they were first made in 1950.
My personal opinion: don't buy a closeout Express with the hardwood stock and plain, skinny rib. I have one like that and it's a great field gun. But the new laminate version with the wider, machined rib is nicer, and worth a few extra bucks. People are paying good money for take-off laminate stocks on eBay, so I must not be the only one who thinks so.
If you can afford it, you won't regret buying a Wingmaster. On the other hand, for a few more bucks you can get the 11-87 Premier semiauto, so think long and hard about what you want.
The Express is a no-brainer purchase, since it's cheap and well-made. Just keep it oiled; the rough finish holds sweat and rusts if you don't clean it. I haven't shot mine in a while, but I don't regret buying it at all. If you move on to higher-end shotguns, the 870 Express is always a good backup gun in case something breaks in the field, or to lend to a friend.
Now the 20 gauge Express and Wingmaster are very similar in weight and swing: light and quick. They both feel great.