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If you're going to have a 2 camera setup, then I'd go with the 400 f/5.6L on one of them, and the flexibility of a shorter zoom on the other. However if you're going to have only one camera body, I'd prefer the flexibility to zoom out, because if you want to catch any "atmosphere" and beach shots, 400mm may be quite a bit too long...The possibility of the 100-400L looks the best solution to me. Again, unless ALL your shots are long shots, get a zoom.
I would agree with this...I have a 400 5.6L, but usually carry it in a backpack with my 70-300L, so I understand the need for flexability.
One more thing to consider- if you plan to use a 1.4TC with the lens, unless it's a 1D body, the 400 5.6L will AF much better than the 100-400, at least that's my experience using my 7D and a non-reporting Kenko 1.4TC.