I've owned a Mossberg 500 (sold it) and currently still have a Rem 870 express. Both are good guns but the Rem holds the edge, IMO. If you do get a Rem, make sure you get one that can shoot 3" shells. Their website will tell you what model depending upon serial # of the barrel.

As to barrel length, I don't really have an opinion. I'm pretty simplistic. Upland birds I like a short barrel (26" for an o/u that I shoot) and just change chokes. For waterfowl, longer barrel (+28") because I'm stationary and able to prepare for the shot, ie. no swinging the gun around.