If you're after speed and quality I'd go with the 70-200 L f/4. It's non-is, so if it's dark you still need to hold steady but it's very sharp, faster than the 70-300's.

Alternatively, if you want the length ofthe 300mm you should consider looking at the 70-300 DO.

Pros are it's a lot more compact than the other 70-300's, but it is heavy and in bright light you get a slight white fringing effect on very bright white things

On ebay 2nd hand these can go quite cheap (the list price is quite frightening)

I've used both and if it's speed and image sharpness, I'd go for the 70-200 f4 L, but when travelling I take the 70-300 DO as the other one doesn't fit nicely in my bag!

the digital picture has reviews of both and sample images so it's worth looking there