Thanks doc. You could just send me your 70-300L for a few weeks, since you don't have room for it.
I stand corrected, I had simply looked at incomplete info (and also don't recall seeing it on Canon's site).
"Image Stabilization 4 stop, 2nd Generation, tripod sensing"
The rental site describes the 100-400L differently:
"Image Stabilization 2 stop, panning mode"
Since it does have panning mode after all, certainly the 70-300L seems like the ideal zoom lens for a full frame camera. Small enough to carry around, goes to 300mm. f/5.6 at the long end, but since you can shoot (or I would shoot) up to ISO 25.6k or more...that makes it not such a slow lens, as it would be on my XXD camera.
Here's a shot I did last fall of yet more deer, with my pre WW2 era 50D, at ISO 6400 (keep in mind that is "H1" on this camera)...with a 270mm f/4 (135 f/2 + 2.0x TC ii), 5 minutes after sunset, hand-held, no IS. The PS-corrected noise (and downsampled size) certainly look smooth enough to me for "web viewing". Opinions vary. Not quite a pro quality shot, but way above amateur