1) obviously, one full stop of light
3) speed to focus.
But this is mostly splitting hairs. This is an outdoor lens and f/4 is usually going to be plenty. And while the 400mm prime is a wonderfully sharp lens, scoring 15P-Mpix on tests by the well respected DXOMark, the 200-400mm neverthelss scores a respectable 11P-Mpix, which ranks in line with the well regarded Nikon AFS 24-70mm (11P-MPix) and just a tad below Nikon's 50mm 1.4D prime (12P-Mpix) so it's no slouch. I'll deal with speed to focus in another post but let's just say here that both lenses are fast enough.
But the 200-400mm has positives in its favour also:
1) it weighs a kilo less than the 400mm prime and is less bulky; this is a meaningful difference,
2) it costs 25% less than the the 400mm prime (enough to buy you a D800 body!)
3) the zoom provides a flexibility that the prime does not, and sometimes this really matters.
This last advantage might be considered something of a dealbreaker for many, including me. Check out the pictures below taken at Brands Hatch last weekend. Admittedly there was harsh sunlight that day, so at 400mm f/4, even at 1/1250 shutter speed the camera is at ISO160, that extra stop is not needed. But more importantly here, as the car came round the corner (picture 1), the 400mm focal length allowed it to nicely fill the frame, but just 3 seconds later (picture 2), 400mm is no good, and to get the shot I wanted required pulling back to 300mm, and even here the car is not in full. If I wanted both cars end to end, 200mm would be the order of the day.
Sometimes photography is predictable, but often it is not. At motorsport, a spin, a crash or a dramatic overtaking manoeuvre will not always happen where bests suits your 400mm prime and I find the flexibility to quickly pull back to 200mm when the action moves to me an invaluable proposition. For sure, I don't expect to be doing too much shooting at 200mm during a day at the track but I will do some, enough to make flexibility a worthwhile ace to have in your back pocket.
Saving a kilo on weight, saving even more on money and having that flexibility to shoot both near and far easily for me outweighs a stop of light and modestly improved sharpness. Unless you are a top pro shooting for a magazine where that stop of light and extra sharpness is the difference between making the cover or not, I'm not convinced, unlike Fro's readers, that the prime here is the better option. The more I use the 200-400mm, the more I like it.
I'll be writing more on the 200-400m in due course.