Canon 70-200mm f/4 L IS vs Canon EF 70-300mm f/4-5.6 L IS for landscape/travel photo?

Since you are planning to take two bodies anyway, have you considered that, instead of two 6D's, you could take a 6D and a 7D?  You could take a 70-200/4, which is one of the most spectacular lenses made in its range, and you would also have, in effect, an equally spectacular 112-320/4 on the crop.

With refurb 7D's going for just a bit over $1k, you could think of it as a 1.6 extender that adds no weight to your bag (given that you are taking two bodies anyway), and I suspect will have better resolution than a 6D w/1.4 extender.

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WLW