One further thing to consider is the tracking of AF for the wildlife use. The 70-200 with the 2x will leave you with a single point which is fine for stationary targets or slow movers. The 100-400 will leave you the full spread of focus points so your AF can still track - sorry I can't remember which body you said you had, this will make more difference depending on body. It doesn't just come down to IQ difference, and I agree with what others have said, the 70-200 with the 2X attached isn't that great to hold.
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