Hi majere2sg,
Like you, I had found myself in the exact same shoes some months back when I was looking for a long telephoto lens to complement my collection. I was also contemplating between these 3 lenses. I have owned and used the 400 5.6L and Sigma 50-500 before, and have used briefly a friend's 300 f4L IS.
Firstly, you have to ask yourself, "What do I want to shoot?" and secondly, do you mind lugging a tripod around? If the 2nd answer is a definite no, then the choice is clear: 300 f4L IS is the lens for you.
The important thing to realise is this: long lenses of 300mm and above (assuming no IS) need a good, stable platform to perform at their best. Else, you pay top dollar and get average image quality if you just handhold the lens when shooting the subject. A tripod, or at least a monopod, is a must. Handholding is still possible to obtain acceptable results, but you aren't optimizing the max optical quality the lens is capable of as the inevitable handshake will deteriorate the overall image quality.
If you shoot mainly animals in captive conditions (i.e. zoo, birdpark etc.) I would recommend either the 300 f4 IS (w/ or w/o 1.4x TC) or the 50-500. The latter is especially useful coz in tight situations you can still zoom in and out to compose a pic nicely. But you would need a good solid tripod + head to go with it.
Sharpness and image quality wise, there is no debate, the primes win hands down, esp the 400 f5.6...its one heck of a sharp lens! :bigeyes: The Sigma 50-500 is no slouch either, and its sharpness is comparable to the other two esp. when stopped down.
AF speed: the Canons are still the fastest, with Sigma lagging slightly behind. The Sigma tends to hunt a little with non contrasty subjects.
Usage with TCs: the 300 f4 is best in this respect. Put a 1.4x TC on it and it becomes a 420mm f5.6 with still respectable image quality. Also possible with the 400 5.6, but u need to tape certain pins in order for AF to work. Else its MF with this combo. Forget abt using the Sigma w/ TCs...the image quality becomes very soft and quite a bit of CA too.
In summary, I would recommend the 300 f4 IS. A superb all rounder lens, has IS, can be used with TCs (esp the 1.4x), and with extension tube, even can be used for telemacro on skittish insects such as butterflies (with excellent results). If you don't mind lugging a tripod around most of the time, the Sigma 50-500 is a worthy contender too, its main strength is the flexibility of zoom.
Hope this helps!