From an L standpoint, I started with the f/2.8 but have both now. I traded my 70-300mm non-L plus cash for the f/4. If I could only have one, no question I pick the f/2.8. If hiking 5 miles or less, the size & weight of the f/2.8 has not been a problem. (Age 66). Unlike another post, I find the AF speed of the 2.8 significantly faster. Both lenses work well with the 1.4X TC III, but of course the 2X TC only works with the 2.8 on most bodies. For BIF the 400mm f/5.6 is drastically better at maintaining focus lock. On the other hand for more general wildlife, the 2X TC is ok, and maintains IS and a close minimum focus distance.