Both good shows. Buffy is one of my favorite shows ever, but I'm tempted to vote for The Shield just because it's a great show and needs the help in this competition. Obviously in a place called Buffy Boards, The Shield is going to have trouble competing with Buffy the Vampire Slayer.
For those who don't know, the creator of The Shield was a writer on Angel for awhile, and has said that he owes Joss Whedon a great debt for teaching him a lot about working in television. That said, The Shield doesn't remind me of Angel or Buffy at all but it's worth considering if you've never seen it before. That and the character of Vic Mackey is really one of the great characters of television. He's both despicable at times, heroic at other points, and occasionally both at once which makes for excellent television. I've only seen the first four seasons (though I basically know what happens at the end of the series), and I have to admit the first few are quite a bit better than the fourth. The fourth is still pretty good though. Give it a shot even if you don't like cop shows, because I normally don't but I like this one.