I typically prefer Remington 870s for my utility shotguns and have a couple - one set up for hunting and a tactical version for home defense, but I've got a Mossberg too and don't really have anything bad to say about it. I'm not as used to its safety position, but that's a training issue rather than any sort of qualitative difference.
If the individual gun you purchase looks good and works well, and you're happy with the controls, I don't think you're going to go wrong with either one. Both have significant aftermarket support, and parts galore, although I think the edge on aftermarket goes to the 870.