What's wrong with the .410 round? Get some of those PDX rounds from winchester, designed for the taurus judge. Awesome defense ammo, and put a butt cuff on it to hold a couple reloads. A double barrel makes a fine HD gun. Their recievers are short, and in the event of a defensive shooting, that gun is now evidence. When the jury sees your gran-dad's ol grouse gun, they're going to have a hard time painting you as a trigger happy gun nut. The short over-all length of a side by side make it handy for a home environment. Also very simple, nothing to stick or jam. Then there's the issue of penetration. Unless your using hollow or soft point ammo in your SKS, your definately going to go through a wall, where your wife/kids/room mate/dog/Xbox is sleeping. Worse yet, your likely going to go through your perp. I keep my SKS loaded with wolf hollow points. I'm not saying the SKS is a bad defense weapon, just that the SxS .410 isn't, either. They both have their drawbacks.
The best gun for defense is the first one you grab.