Bikersbeard was suggesting the 7D was worse than the 5D... The AF on the 7D is more up to date, my question really was is that due to the lower light collecting ability of the lesser lens used on the 7D
The 7D AF is very good. It is designed to allow tracking and keeping focus for fast moving subjects, certainly much better than my 5D MK II.
However, in low light, a FF camera with its larger AF sensor and larger main sensor gathers more light and will function at a lower light level. Its the same for other FF camera bodies.
This is assuming that we are comparing the cameras with the same lens under the same situation. Using a f.1,4 lens on a 7D versus a f/5.6 on a 5D MK II might give opposite results.
Remember, we are talkiing about photography where it is almost totally dark. The kind of low light where a 7D would not get a good image even if it did autofocus. Night clubs, Theatre, very dark.
From my testing, the 7D can focus quite well in low light up to the point where the sensor only produces very noisy images that you would not like anyway, so I'm not knockiing it, it is well balanced. But, when my 7D quits for lack of light, my 5D MK II is still going strong.