I'd be willing to bet dollars to pesos you will get about 80% saying 4.8is... you are on an X5 forum afterall.
The SRT8 is great for straight line performance, but lacks the fit and finish of the interior and exterior as well as all the great handeling characteristics that make the BMW really shine through what it is.
The Rover is a decent vehicle, but can have even enough problems to make the BMW look much better, and still doesn't have the handeling/fit and finish of the 4.8is.
BMW wins this comparison in my book.