canon's focusing points compared to nikon's

I was wondering when selecting an off center focus point on a camera such as the 40d, how the focusing system works. Like how it decides what to focus on.

As opposed to how off center focus points on a nikon d200 would choose its focus.

And suppose i'm using a 70-200 2.8 on both cameras.

Which works best for say portraits and wedding photography where the bride and groom are moving around?

I can't really see how anybody is likely to know this, Scarter - far too many variables involved...