No matter what anyone here says, go pick all of them up with all of the different options and shoulder them. Both the Nova and the 870 are solid, reliable guns, but both fit differently. Close your eyes, mount the gun like you're ready to shoot, then open your eyes. Is the sight plane perfectly lined up with your eye so that you shoot wherever you are looking, or do you have to adjust your aim to make it line up? On a gun that is too short, you will see the bead and part of top of the barrell when you open your eyes. On a gun that is too long, you can't see the bead. When it's just right, you only see the bead. And remember a little too short is better than a little too long... because you will be wearing a coat and extra clothing.
Accurate wingshooting is all about instincts. If a gun doesn't fit properly, you have to think about adjusting, and you will miss more often. When both guns are reliable, choose the one that fits best.