If you have to decide between a Remington 870 and a chinese copy of the 870, get the genuine Remington. If you really wanted a shorter barrel, you can buy an aftermarket replacement.
I would, however, recommend the Mossberg if you're actually going to USE the gun in the field and on the range, rather than just dressing it up with accessories. I've got around 1000 rounds of various 2 3/4 and 3in ammo through mine, and it has cycled everything flawlessly. The interchangable chokes in the 28" vent rib barrel work well, and you actually get 3 different ones with the gun. If I am correct, you only get 1 with the 870 express. Fit and finish on all parts is good, especially for a 300 dollar gun. As well, I never did like the slide release on the Remington. In case you're unaware, the slide release on the Mossberg is located behind the trigger guard where it is very easily accessible without moving the shooting hand, while the Remington slide release is forward of the trigger guard, kind of out of the way and requiring the shooting hand to be moved away from the firing position. As well, the safety on the Mossberg is a sliding safety switch mounted on the tang (One of two plastic pieces on the gun, and there is a 'mil spec' metal replacement if you so desire). This safety is much friendlier to lefty shooters and can be more easily actuated with the hand in the shooting position compared to the crossbolt safety of the 870.
I've used my Mossberg 500 for clays, upland, and ducks, it has performed flawlessly since I've owned it. It's the only firearm I have that I bought new (So far!

), its simple to disassemble and clean (even if you need three hands the first couple times you reassemble it) and I'd buy another tomorrow.