Defense at 20 yards...
Defense at 20 yards? If you're speaking in generalities, it doesn't matter what gun you have. Whip out your teeny tiny Kel-Tec P3AT. Fire a warning shot into the air.
A man with burglary on his mind will get the hell out of there, fast.
Now, maybe you're not speaking in generalities. Maybe there's a reason why a determined person would want to approach you even after your obvious display of armament. If that's the case, I seriously doubt he would be walking across open field without taking cover. I also doubt he would be without some sort of long gun. I'd also be worried, if I were you.
If, for some reason, the above scenario is taking place, I'd expect there to be more than one of the opposition.
That said...
I'd go with a Saiga shotgun (automatic shotgun built on the AK action) or a military-style repeating rifle (AK-47, AR-15, etc). Everyone has their preferences. I dig the AKs myself.
At the very least, pick up a Mossberg 500 (7+1 shells of buckshot) or a decent carbine. Kel-Tec makes a nice, folding 9mm/.40 carbine that can fold in half and fit in a standard briefcase, which can be bought in several variants that accept S&W, Glock, or Beretta magazines, which means you can swap mags with a pistol.