I have both the 400mm & the 100-400mm. The 400L is definitely a better lens for both sharpness and AF speed. You did not say what the intended purpose but for birding, you don't really need IS if you keep your speed up as Inkista mentions.
The 100-400mm is also a good lens for general purpose and I have used it for motor sports as well as walkaround. Doesn't AF so fast for birding or wildlife.
Allen...
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Gear: Canon 1DMkIII | Multiple Canon lenses
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