The 300mm 2.8 is way out of my budget, but the F4 is affordable and a good lens. As I've read other places, the 300mm F4 combined with a 1.4x extender will work ok, but might not work with the 2.0x? I have a 5D Mark II
You can use either the 1.4x or 2x with the 300/4, technically. But with the 2x, you have a 600mm f/8 lens - that means no AF on your 5DII (1-series bodies can AF with an f/8 combo, the 5DIII will be able to next April, assuming the firmware is released on schedule). The 300/4 will take a bigger IQ hit with the 2x.
Thanks for the info! I think the 300mm alone will be enough for me, maybe a 1.4x if I feel the need for longer focal length or just putting it on my 50D. The 400 F5.6 could be an option, but it lacks IS. But this is a dilemma for the future (just bought a used 85L so I'll have to start saving up again )