To take concert photos, would you need a camera body that is fast or a lens that's fast ?

It's ideal to have both a fast lens and a camera body that performs well at high ISO. That being said, if I had to choose between the two, I'd care more about getting a fast lens. Noise can be fixed when retouching the photos, whereas, bad exposure (especially when underexposed) will be harder to fix. Oh ye, one more thing, shoot RAW!