I have an 870 Express. I compared it with the Wingmaster version yesterday. I'd managed to gum mine up with the wrong kind of oil yesterday, but under normal circumstances, it is very similar. The Wingmaster slide is a tad tighter feeling, and the trigger assembly has a metal frame -- Aluminum alloy, I think -- whereas the Express has a plastic trigger assembly. Don't be turned off by the word "plastic" -- this plastic like Glock plastic, not plastic like toy plastic.
The Wingmaster has a nicer rib, a nicer front bead, and a middle bead which the Express lacks. The Wingmaster finish is polished, whereas the Express finish is rough. The Express is prone to light surface rust from sweat and water, but the flipside is that said rust wipes off with Rusthunter or Break-Free and a rag.
The Mossberg also has a nicer rib than the Express, and the Mossy comes with two beads like the Wingmaster. IMO the slide doesn't feel as good as an 870, but it's not at all bad. The receiver is aluminum alloy, so the all-steel Remington can take more abuse in the form of heavy loads, but the Mossy doesn't rust. It will get scratched up more easily, though. I've never had a "scratch" on the Express that didn't disappear with some gun oil and a rag.
All told, if I'd have known how much I'd like shotguns, I would have gotten a Wingmaster first, and an Express for throwing in the dirt or boat.
Between the Mossberg and Express, I'd say it comes down to fit and feel. What feels good in your hands? Their stocks are different; the Mossy has a fat wrist. If you have small hands, it might be a problem, but otherwise, you might like it.
The Mossberg does come more fully-equipped than the Express: it has three chokes vs. the Express' single choke ($28.50 at Wal-Mart for the "upgrade kit" with the other two, a nicer wrench, some oil and a box). It appears to come with sling swivels, though that doesn't appear in the specs. It will cost you between $13 and $30 to add them to the Express, if you want them. (One upland hunting season without them, and I'm ordering some! If you hunt in boots, the sling swivels are a real plus!) Mossberg also sells spacers to change the stock drop, for better fit. An 870 happens to fit me as-is, so I don't care, but otherwise this is a neat thing. The only thing that I would recommend is, if you get the Mossberg, remove the rust-producing wooden magazine plug and throw it away! Or keep it just long enough to use it as a gauge so you know how long to cut an aftermarket plastic one (a few bucks from Wal-Mart or your favorite vendor).
Note the difference in trigger reach, grip angle, etc. between the guns. That's one reason to go and feel them for yourself.
I'm an 870 fan, and I may never buy a Mossberg shotgun. But it's hard to argue that someone else shouldn't.
