On Photozone, the rez of the 100-400 @ 400 looks comparable to the 300 w 1.4tc, and the 400 f/5.6 prime. I would hazard to say that the 100-400 would be much more versatile.
http://www.photozone.de/canon-eos

1minute- does your wife find that she is often at the 400 end of the zoom?

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