Renting a fast prime (200mm or 300mm) for the olympics seems like it would not be versital enough. The athletes or action will not be stationary (move closer and farther away), and your distance from the action from event to event is going to be unpredictable. So it seems to me a zoom is needed to cover a wider range of distances from the action that you are likely to encounter. Personally, I would go for the 70-200mm f/2.8L IS II, and resort to the 1.4X converter outdoors if you need more reach (more reach may not be needed indoors).

I have the 24-105 and 100-400, and neither is likely to be good indoors (as others have stated as well). And if they are not good indoors, then I doubt the 70-300L would be any good either.

Given a choice (money not an object), can anyone here seriously say they would pick a 24-100L, 70-300L, or the 100-400L over the 70-200mm f/2.8L IS II ???? I know they all have different uses, but if I could only get one lens... it would be the 70-200 II if I had the money.