The 5D Mark II's biggest strength is its full frame sensor, which as mentioned is mostly a hindrance to you -- you'd need to buy bigger, and thus more expensive, lenses to achieve the same framing you would have with an APS-C sensor (though I will add 21 MP gives you plenty of room to crop). Secondary strengths include a great viewfinder and amazing performance at 6400 ISO and below, with crystal clear 1600. Don't neglect this -- if you want to take a high shutter speed shot at night under field lights or in an indoor stadium, you need decent high ISO. No flash in the world will help you in this environment.
Both machines have the same AF points. I've heard tales of the 50D has superior AF, but the 5D's AF is nothing to sneeze at, and it'll focus in damn near darkness, which is a huge plus.
Honestly, these days any camera in Canon's line, including the consumer models, would fit your needs. I say buy a GREAT lens, then buy the camera that's most comfortable on your hands and wallet. Above 12 MP, the lens is your biggest limiting factor anyway.
I found the 5D to be worth the money, but I do almost exclusively available light and landscape work. It's a heavy, heavy camera.