Birding: 7D and what lens?

Although I'm well invested in m4/3, I am not totally pleased with the GH3 and/or OMD (yes, I have both) with the 100-300mm (200-600mm effective) lens.  This gives a very small package, with reasonable but not great performance.  So I thought I would up one sensor size.  FF is out, that is just too much to carry.

I'm thinking of a 7D body since it is such an excellent machine.  But I need some help with lenses.

I could go for the 70-400mm, with the crop factor this would be about 100-600 -- but I am not sure I would like the push-pull focus and zoom.

Or, the 70-300mm f4-f5.6 IS USM UD lens with a 1.4XTC.  Smaller package, little better total range.

I would appreciate any comments or suggestions.

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= John
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