I thought long and hard about the Canon 100 – 400 L before I settled on the Sigma 100-300 F4. The Canon 70-300 seems to be on par with the 100-400 here:
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=738&Camera=453&Sample=0&FLI=4&API=1&LensComp=113&CameraComp=453&SampleComp=0&FLIComp=5&APIComp=2
In the end, I got the Sigma at 1/2 the price of the 100-400 and I’m happy with its IQ. None of these are going to be inconspicuous, casual tele-zooms. They tend to get people’s attention. At 2.3 lbs the 70-300 is slightly lighter than my Sigma’s 3.2, but either way you’ll tend to consider the need to carry it on a long hike.
AF speed will be a consideration if you trying to follow the subject. Some super zooms have very slow focus speeds.
Don’t forget TCs, they aren’t the best solution especially with zooms, but if you’re carrying your gear and water on your back all day, a 1.4x TC can make a long lens out of something light if you have the aperture to work with.
(that comparison site lets you add a TC on some lenses)
