My 870 experience=old Wingmasters vs. new production 870s (not new Wingmasters)....so maybe not relevant to the question...but..
I'm no expert on 870s (or anything else for that matter) but I have been shooting them for almost 30 years. I've owned old Wingmasters, new Expresses, and currently use an 870P with a 28" VR barrel for hunting/clays. In my experiences with them~treat them right, they'll all go "bang" when you need them to. Period.
The only REAL issues I've seen on new Expresses tend to be things people blow way out of proportion..with one exception~SOME chambers on 870 Expresses leave the factory kind of rough. They tend to cycle good quality ammo with no issues...but cheap/lumpy stuff like Winchester white box can get hung-up after firing. Some people attribute that to the single extractor, some to rough chambers and on one gun I worked on....a burr on the shoulder the rim sits in directly opposite the extractor. In all cases, a little Google work and 20 minutes of TLC remedied it. I used a Dremel to remove the burr. I do hear allot about the poor finish on Express guns and yes, they aren't polished/deeply blued like old Wingmasters. Not as nice to look at, and more inclined (in my opinion) to hold moisture. BUT...the biggest complaints seem to come from guys who put these guns away wet, then complain they're rusty when they take them out of the case next time. Would the same guys put a nice Wingmaster away wet? Yeah...don't think so. I think the cost of the Expresses seems to invite poor treatment by some guys..who subsequently yang about the finish on web forums. I've removed allot of stuck choke tubes from guns like this.
Where I think I see the biggest differences (besides aesthetics, smooth cycling, etc.) is when you tear them down to dry them out, clean them up, work on them, etc. No question, the old Wingmasters (and my new 870P) have nicer internals, fewer...or NO nasty/sharp edges, etc. I've never worked on an Express TO smooth things out (they run fine new) because they'll honestly smooth themselves out after enough shooting.
The only hole in my 870 experiences so far...I think...is with NEW production Wingmasters. Since 2-3/4" shells do everything I need them to do, and, since I have that 870P with spare barrels that cover all bases...the need for a snazzy/polished new Wingmaster just isn't there.