OK Dave, I just shot the D7100 with the AF lenses that I have, as mentioned in my first reply. Only static targets but was rapidly moving from targets at a couple of mtrs to infinity and all ranges in between. To be honest all focused at speeds to close for me to really say which is faster. We are off to lunch in an hour or so. I'll take the D7100, 85 1.8G, 105vr and DC105 and try shooting motorbikes on the highway across from the restaurant. Only problem is that I will have lots of light but there will be buildings as BG on the other side of the road.
Next time I go out in the early morning for birds I will take same set up and check low light. Very probably tomorrow morning or Tuesday.
It may come down to which FL you really want, how much you want to spend and what other uses you have for the lens. Most sports shooters say that the DC135 f2D is too slow for indoor sports but I am now wondering about that.
I will get back to you in the next couple of days. Maybe Jakov can shoot some moving targets in Scotland, with his new 135 f2, and see what he thinks
I actually shoot more primes than zooms these days but would never shoot soccer without a zoom on one body.
Edit: I will do some High and low speed burst shooting and see what happens with tracking . That could be the differentiating factor
Tom