We're looking into options for a reasonably priced telephoto lens for capturing Dog Agility outdoors on a 7D.

I have it narrowed down to the 70-200mm F/4L IS USM and the 70-300mm F/4-5.6L IS USM.

The 200/300 primes are out, as they just aren't versatile enough. We love our primes, but we need a zoom here. The 70-200mm f/2.8 USM is out, as it doesn't meet my wife's demands for sharpness. Similarly the F/4 USM is not sharp enough. The F/2.8 IS versions are out of our price range and are too heavy. The 100-400 is also too heavy.

70-300mm F/4-5.6L IS USM is as fast, or nearly as fast as the 70-200 F/4 over the majority of the zoom range, only trailing significantly after 155mm, but that's offset by the extra 100mm of reach, so it sounds ideal, except for a few comments in Bryan's review. He mentions it focus-hunting more than other lenses of it's class. Is this commonly occurring thing I should be concerned about? He also noticed it front focusing in cases. While it's impressive for a lens to outpace the action, what we need is rock solid tracking.

Does anyone have demanding servo sample images, equivalent to what we might see in agility, that they could share crops from? Something small, fast, moving towards the lens. Perhaps some winter sledding or something?

If anyone can offer some insight into either of these lenses, or other lenses we should be considering, that would be great. If anyone has used both of these and can comment on their relative servo performance, we'd appreciate it.