I posted a thread similar to this one a couple of weeks ago, asking about the best HD load for a 12ga. I got a multitude of responses, but the overwhelming majority tends to agree that buckshot is the way to go.
Sure, birdshot will put a hurtin' on a BG, but you're going to have significantly less penetrating force, and consequently you might not bring him down right away. I've read several stories of intruders getting hit with a couple rounds of birdshot, and still being completely concious. Hurting like a mofo I'm sure, but otherwise "ok" considering they were just hit with a 12ga round. In one case, the BG was able to walk to the ambulance when it arrived! Now, that may have been a very rare occurance, but if I have to shoot someone, I don't intend to allow them the opportunity to get back up. Getting up means still being able to reach a weapon and attack me and my family. Not gonna happen on my watch.
In order to penetrate someone far enough to ensure a lethal hit, you have to take the risk of going through wall or two. If you are trained and familiar with the weapon and the ammo you're using, that risk is minimized quite a bit. I'd say that 00 or #1 buck in any flavor is your best choice. I'm no guru, I'm just going off of what I've read/been told by people far more experianced than I am....I can tell you this though - Federal 00 buck will be in my HD gun's mag, with a few extras and some slugs waiting in the sidesaddle.