I have both and as many have, I've agonized over the issue before getting the 2.8II. IQ is pretty much the same on both, obviously from F4 onwards and obvioulsly you get more flexibility with the 2.8 for the reasons explained above. That extra stop is a great thing to have, whatever high ISO cameras exist now, it is always a plus.
Asuming money is not a big driver in the desicion, this was my rationale:
If you consider/want to be able to shoot (also) some wildlife without going for a dedicated lens, get the 2.8 that can use the 1.4 and 2x extenders, big versatility value here and IQ won't dissapoint, even with the 2x it is more than acceptable with good light. You get basically a 70-400 (f2.8 - f5.6)
If portrait is your thing, I'd say get the 2.8II, here I think there is a real visible differnce btw f4 and f2.8
If you value more the size and weight, then the F4. Significant weight difference here and the F4 is still more than usable with the 1.4x by the way. Being limited to 280mm in such case you get "only" a 70-280 (f4 - 5.6)
In my case, I value more the possibility of putting the 2x extender whenever I need to than the weight penalty while travelling, therefore I'm selling the F4. FInally, the 2.8 comes with the tripod mount ring for free, a night and day change whenever you use the tripod, logically.
Good luck with teh choice!