100-400 vs. 70-200/2.8L IS mk1 vs. 70-200/4 IS

[barba]

I own the 300 f4 IS and I would certainly add it to your list to consider. The 1.4x extension certainly increases its utility, but it does seem to slow down the AF somewhat. The IQ is still great.

[Jordan]

I know from using it (just no photos online right now, sorry) that the 70-200 f/2.8 L IS USM with 2X extender isn't as good quality as the 100-400 by itself. I won the 100-400 and though it is a little slow here and there, I would highly recommend it. Canonrumors.com has said they've discontinued it though... will something better come out perhaps? Maybe wait for it's replacement or get it now while you still can? Tough choice.

[tancanon58]

I sold my 70-200 2.8L IS to purchase 100-400 4.5-5.6L IS which has more reach and it is fast focused with high ISO body like 5D2 and 1D4.