I guess my priority is getting range, and good quality, in a fairly compact package.
I tried a few lenses (and the 70-200 with a 1.4 TC) at various shops today.
Ed, you're right - the 100-400 is smaller than it looks in photographs, even at full extension. It's also lighter than my 70-200/2.8 IS. I actually got along great with the 100-400's push/pull zoom, it just seems to fit. Focus was snappy in single-shot and servo (the store's in an urban environment, so I could step outside, shoot some cars coming at me, as well as pan left/right). Really enjoyed the handling on it, and the price (new) wasn't bad.
Gave the Sigma 150-500 a tryout as well, but that really is a monster of a lens - a bit too big, a bit too heavy, the zoom ring is far more annoying to use than the push-pull, and 150 is quite a bit longer. I can see myself walking around with a 100-400, but this one...heck no.
The 1.4 TC on the 70-200 gave me good length, but the zoom range isn't as much 'fun' as the 100-400. In terms of handling, the 100-400 gets the nod, by a hair, followed by the 70-200+TC.
I'll be trying the 300 on Friday,see if I get along with a prime at that focal length - I love my shorter length/medium telephoto primes, some of my best shots have been taken with those, but the 105mm (on a crop factor, admittedly) does nudge the edge of my comfort zone in terms of being a little on the long side. Took a good long while to learn to use a 170mm-equivalent prime, although I did learn to love it in the end.
The weird thing here is there seems to be a relative glut of 300 F/4 lenses on the used market (5 with IS, 1 without at last count on one equipment buy/sell site, none on the two bigges craigslist equivalent sites), while the 100-400 is in far, far shorter supply (1 currently going, for a price that's so close to the new price I wouldn't bother...)