B4 for me, I've only had a 'go' on a bk2, but I dont think it gets better than a B4 for the normal racer at the moment, its just a great car.
The BK2 is used by chris long, and. I cant think of anyone else off hand that uses it. Maybe paul rotheram but thats only because he broke his hybrid !
Everyone else seems to use a heavily modified version of the car - similar to the new XXXCR thats on its way. That looks a lot better car, nice plastic parts !
At the end of the day, there probably wont be that much in the two, performance wise. Losi always have more expensive spares, so if you are going to be breaking things then its something to consider.