5D Mark II is a full frame sensor camera, and thus would be expected to perform better in low light. But low light and action photography depends more heavily on the lenses than the camera bodies.
You are asking to buy fast lenses (higher aperture rating), such as those with f1.4 to f2.8 to allow faster shutter speed (helps greatly with actions and low light). And each and every individual lens performs very differently, you need to spend time to read reviews and money to invest in them.
Do pay attention to:
- Cropped senors will have an effect on focal length. 50mm is considered "normal human eye view" on 5D Mark II but will look too "zoomed in" on 7D (which requires you to back up farther). 50mm x1.6 = 80mm, and the famous Canon 85mm portrait lens will become 136mm on 7D. It's not bad, but it requires more distance between you and your subject.
- As mentioned above, 5D Mark II does give you a boost in low light performance due to its full frame sensor.
- IT IS THE LENS! Buy the camera bodies for the features, but it is the lens that makes the majority of the image quality. Well, to be correct, your skill also matters as much as your tools, if not even more. But let's just assume you do have adequate skill to pull them off.
