I recently had a similar desicion (70-200 MKII) and went for the 100-400, not for the extender (III-version), for the following reasons:
- If looking at the price (Extender rougly costs 1/3 of the 100-400 in Germany), the value of the 100-400 is much better. I can resell the 100-400 for a resonable price, as soon as I can afford the new 200-400.
- For my type of work adding the extender to 70-200 on the fly takes to long, is to complicated and dirt might get into the optical system.
- I'm quiet pleased with the optical quality of the 100-400 (on a 5DII), it's much better then 70-200L IS Mk I + 2x extender (almost as good as the 70-200L IS Mk II + 1.4x) . AF is fast (on a 5DII).
- Depending on which sport we are talking about (outdoor) the 100-400 might be ok/fast enough. For indoor sports both the 100-400 & the combination above maybe not are the first choice regarding speed/aperture, but they at least are resonably priced.
Get the 100-400, it's a good lense and the far better investment.