Difficult decision, Mike.
Maybe this helps: the Q7 is/looks bigger than the X5 M and performance is also MUCH worse. From 0-200 kph, the Q7 V12 needs 4 seconds more than the X5 M, this already would be an argument against it.
Still: while shopping/looking for a new family car back in December, I looked at the Q7 V12 too. It has some advantages when you have kids and a lot of luggage but since I don't know your family status, it is difficult to give a good advice.
As a daily driver, the Q7 V12 should be great because of the monster torque and the low Diesel consumption compared to the higher fuel consumption of the X5 M. On the other hand, the X5 M definetely feels much sportier and lighter but it is mainly due to a different chassis setup.
The X5 M and the Q7 V12 both have the advantage of being "understatement" cars, they don't look as expensive as they are. In Germany, the Q7 is considered to be a luxury SUV like the Cayenne, so this would be a no-brainer over here if I had to choose between the BMW and the Audi.
You still have the Cayenne Turbo ? So why would you want another SUV ? The novelty factor kicking in ? 
I also don't know about rebates in your country. Not sure if money is a real issue for you but even if it isn't, I would still want the best bang for the buck if I could. The X5 M deal I got from my dealer was a pretty sweet deal, not even close to what my Audi dealer offered to me or actually any other brand dealer. If it is the same or similar in South Africa, going for the X5 M might be a good idea.
Good luck with your decision, life is tough. 
Btw: I'm surprised to hear that you liked the exhaust/engine sound. When my X5 M arrives (hopefully anytime soon
), I'm going to evaluate the exhaust/engine sound again. So many people seem to like it, I need to pay more attention to it.
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RC (Germany) - Rennteam Editor 997 Turbo, BMW X5 M (03/2010), BMW M3 Cab DKG, Mini Cooper S JCW