The 70-300 f4.0-56 USM IS, it's an ok lens, decent performance, but over the 200mm range it starts to soften a bit, at least that's the impression it gave me. The 70-200L, either the f2.8 IS or 4.0 IS models are entirely different beasts altogether.
I was undecided between the f2.8 (IS) and the f4.0 (IS). After having tried both, well, the extra stop of the f2.8 is nice, but the extra weight isn't (nor the price difference), so i got the f4.0 (IS). You could get the non IS f2.8 for the same price of the f4.0 IS, but if my memory serves me right, this older model has no weather sealing, something which i found quite handy on a couple of times already. The IS... the IS is nice, but don't forget to disable it if you're using it on a tripod. It would be nice to have in the camera (whatever camera) an option to IS on/off via a switch of button, specially if you're focusing manually in live view with magnification (perhaps there is and i haven't found that yet).
The lens performance is astonishing. Sharp images, good contrast, good color rendition. It's somewhat prone to flare, specially if you shoot at night, but then again that's why you have a lens hood.
The only negative point in the lens is that it's not included a lens ring mount, and although the lens isn't really heavy (unlike the f2.8 which starts to make itself felt after an hour or two) if you plan on using it a tripod, get the lens ring mount and avoid stressing the lens mount in the camera.
The price, you get what you pay for, i don't regret a bit having bought that lens, it's worth every cent.
If you have the chance to try both the f2.8 and f4.0, try it, see for yourself the results.