70-200 f/2.8 L USM paired with canon extender 1.4x or 2.0x ?

[Joel Eade]

You have good advice already but I will add that after extensive research and personal use the Version III 1.4 isn't much better than the Version II however... the Version III 2X Converter is alot better than Version II.

I have used the Version II 1.4 TC on the 70-200mm f/2.8L many many times and it works very well....it focuses fast and there isn't much loss of image quality. I just acquired the Version III 2X Converter and have not yet tried it on the 70-200 but it does work well on my 500mm f/4L and other users report good results with the 70-200mm f/2.8L as well.

I had Version II of the 2X and sold it because there was too much loss of IQ. It really slows the autofocus down on the 70-200 as well.

As for lens choice, if you plan to shoot alot at 400mm especially birds and birds in flight then you will be happier with the 400 f/5.6 prime lens although you lose some versatility. It will acquire focus much faster and is probably sharper than the 70-200 with a 2X converter.