I own a mossber 500 (12ga.) and have had no troubles with it at all, it has been a very nice gun.
However, I will say that I do like winchester shotguns as well. In the land of the pump-guns, they are all good (provided they're from a reputable manufacturer) but the reason I like the mossbergs over the 870's is because of the location of the slide un-lock, which is in the same place on the winchesters IIRC. I also like how the winchesters handle, but the mossbergs fit ME better.
Shoulder them both, and decide from that, because it doesn't matter how good a gun it is if you aren't comfortable with it.