Keith Reeder , Sep 15, 2012; 04:45 p.m.
I'll sum up the forthcoming long and almost certainly increasingly bad-tempered post (this subject - "which is sharper, the 100-400mm or the 400mm f/5.6?" - must be one of the most argued-over questions on the internet).
Some people will say that it's impossible for a zoom to be as sharp as a prime and will therefore recommend the prime, especially if they've been exposed to a less-than-perfect version of the 100-400mm (they exist).
Others, who know better and who have (or have had) prime-sharp 100-400s (yes, they exist too - and I've got one myself), will say that there's no sharpness disadvantage to the zoom, and a lot to recommend it from a versatility point of view.
The prime is a bit faster-focusing (although this advantage can be somewhat reduced by using the focus limiter on the 100-400mm); the zoom has a minimum focusing distance which is shorter - to a worthwhile extent - than the that of the prime.
And of course, the 100-400mm has image stabilisation, which I consider essential, and which can in itself make all the difference to sharpness in the right circumstances.