300 f2.8 IS L II plus 2x Mk.III better than 100-400 f4.5/5.6 IS L?

birdbrain wrote:

I probably should have made myself a bit clearer.

A combo I use a lot is the 100-400 on a 7D, having got myself a 5D3 I have been looking for a lens and converter combo which would give me a similar amount of reach. The 7D and 100-400 is equivalent to 640mm while the 5D3 with a 300 f2.8 and 2x would be 600 so very similar. While with the 7D I would have 960mm, while shorter than the 500 + 1.4x 7D combo I have it's not bad.

I am looking at downsizing, the new Mk.II 500 is more than I want to pay and although lighter it would not be like the 300 f 2.8.

Hopefully I have made myself clearer?