1. #7 to #9 birdshot is for clay pigeons. I'm going pheasant hunting later this year and I am not going to use shot that small. For pheasants...
2. For home defense, I wouldn't go with either a SxS or a O/U. A GOOD O/U doesn't really exist under $1000. There's the imported Turkish O/Us, like the CZ shotguns, but they're not completely reliable like say an 870 Wingmaster. As for SxS, its got a 2 shot capacity. Granted you can get faster shots off than a guy with a pump, but if you had a semi auto, you'd have that fast ability and more capacity.
3. It depends on where you live. A cloud of buckshot or birdshot will take out a wall and windows and can go into someone else's yard. 12 gauge birdshot normally has a maximum effective range of 60 yards. Because of this, don't be afraid to use a rifle or pistol for HD instead. Shot won't just stop because you hit the target, there's a lot of it and it'll travel. It might even hit the people and pets you're trying to protect, whereas one bullet from a rifle or pistol will hit only what you aim at and maybe the stuff behind.
4. If you want a slug gun, get an 870 and put a slug barrel on it. Or get a bolt action slug gun. Or an Ithaca Deerslayer.