I have the 70-200/4L IS, and am considering switching to the 70-300L.
My reasons are, i would sell the 70-200 and my 300/4L IS, and go to the one lens.
The 70-300 still gives you f/4 from 70mm to ~100mm.. but the 70-200 w/1.4x tc knocks you to 98mm @ f/5.6. 300/5.6 would give me the equivalent to the 70-200 w/tc, but no swapping TC in and out! I love the 300/4L IS, and having 300 at f/4..and the ability to get 400mm with IS(1.4x tc.. for 420/5.6) and AF, but the purple-fringing gets on my nerves a lot. It would also lighten my bag.
Cons for me, and a big one, is the extending with zooming, and consequently more chance for internal dust, plus possible zoom-creep or zoom-lock. Also the variable aperture.. it doesn't bother me on my 15-85 as a walkabout, but with a long lens, and say a bird flies by.. i don't want to have to check aperture in that split-second. And last but not least, issue with a tc. I want the ability to have 400mm with IS at the very least, and with AF would be better.