As has been said, no one can advise you as to what the best brand of ammo is for your particular gun. Shotguns can be very funny about what they like and don't like. For example, my dad has a Remington 870 Wingmaster 20ga with a smoothbore slug barrel. As far as plain foster slugs are concerned, it shoots Winchester the best, Remington OK, and won't shoot Federal worth a darn. I, on the other hand, have a Remington 870 Express 12ga that shoots Federal the best, Winchester OK, the Remington the worst. Patterning buckshot is just as complicated, if not more so, than how accurately a gun will shoot slugs. As several others have noted, the only way to find the answer to your question is to buy a variety of ammo and see what patterns the best. I personally have had good luck with Winchester 3" #1 Buck and Remington 3" #4 Buck, but that's in my gun and not yours.
As to birdshot vs. buckshot: I have no doubt that, at very close range, birdshot can be quite effective because the pattern has not dispersed much and basically acts as one large mass. However, birdshot patterns typically disperse and shed velocity fairly quickly, especially from the IC or cylinder bore chokes of most HD-oriented shotguns. When the distance is measured in yards rather than feet, the small pellets of birdshot do not penetrate particularly deeply and are more apt to cause a gruesome-looking but superficial wound. The larger pellets of buckshot still have enough mass to penetrate adequately even after the pattern has dispersed quite a bit. My personal preference for HD ammo in a shotgun is BB at the finest (and then only in my 10ga) and #1 Buck at the coarsest. While 00 Buck seems to be the most popular loading, I prefer the the slightly finer shot because it gives me a greater payload of pellets.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar