What would the opinion be amongst those of you who have (or have used) both the 70-300L and one of the 70-200 f/2.8 variants? This is, to me, a tough question. One of the things that I love about the 70-300L is that it can stand up in most camera bags and thus doesn't take up any more room for travel. I am about to leave on a cruise and will be packing the Tamron 24-70 VC + the 70-300L as it has become my go-to kit and can fit in a sling bag.
Moving to a 70-200 f/2.8 variant changes the rules for travel. But as an experienced event photographer, I would certainly say in that arena a 70-200 f/2.8 is perhaps the most valuable tool a photographer could have.