I want an L lens and can afford either the 70-300 or the 100-400. Which one?

I was thinking about this question and some of the answers yesterday as I was trying to photograph sea hawks feeding with my 100-400. Not enough reach, not nearly enough. If you're in a bird sanctuary or if the birds land right in front of you as the chap above me says then a 24-105 would be all you need. I'm only a part time amateur and although I don't have the 70-300 I don't see it as a lens to replace either my 70-200f4L IS or my 100-400f4.5-5.6L IS, but I do believe it would slot nicely in between the two. I shot a footy match from over the fence with my 70-200 + 1.4 II extender last year, this year I did the corresponding match from the same position with the 100-400. Big difference, and as you mention sports and birds I strongly suggest you rent a 70-300 first, then the 100-400. Don't let them kid you the extra 100 will be the clincher. Not sure if the 70-300 comes with a collar, the 100-400 certainly does and although I have no problem carrying it around you can always use a monopod if the weight is a worry but there is only an 11 oz difference. Some mention weather sealing, not a problem, I never take my f4s out in overcast conditions to begin with.