i think theres a case for both venues...wembley, who doesnt want to go to wembley?? ask any1 in the world to name 1 stadium, i reckon wembley would be 85% of peoples choice, whatever sport you play/follow, you would never pass up the oppurtunity to do what you love there?? The riders would surely love it? Plus, bringing the gp to london would surely bolster the 45,000 crowd, i reckun u'd get nearer 55-60,000...?? (maybe optomistic, but with heathrow/gatwick/luton/stansted, maybe u'd get a bigger following from abroad?? plus train links etc etc...)
As for cardiff, i felt the stadium was a bit run down, (mucky floors, crap pa system) for a relativly new stadium...However, as a venue, its ideally placed for those wanting to make a weekend/week (ask bagpuss!) out of the event, plus the stadium is located centrely in cardiff, allowing you to make more of a day of it when you get there...
I'd prefer wembley, but as a football fan, maybe im biased....?