I saw someone's post on indoor agility shooting with a 5DmkIII ( http://www.fredmiranda.com/forum/topic/1103203 ). Shooting @ f/2.8, they required an ISO of 10000 to get 1/640s, which really, is the lower end of acceptable shutter speeds for dogs. Considering these parameters, I don't expect to shoot anything indoors. Having the f/2.8 lens won't make indoor shooting viable.

I'm leaning towards the 70-300L, as from 70-155mm the aperture (thus speed and blur) are the same, or just about the same, as the 70-200 f/4L. On the last 45mm there is a noticeable difference, 2/3 of a stop, but that's offset by the extra 100mm of zoom. We'll have to head out to a store and see how both lenses feel. Perhaps my wife will find them all too heavy, and we'll get the 70-200 f/4L non-IS (for versatility) , or one of the primes (for speed and sharpness) in the end.

Thanks to everyone who contributed, even if I am exactly where I was at the start of the thread.