I bought a 70-200L + 1.4 extender combination about 18 months ago and have been very pleased with the 70-200 and reasonably pleased with the two together.
But I think that having read the reviews, if chosing today, for almost the same money, I would probably go for the 70-300L.
The main problem with using the convertor, as mentioned above, is that as soon as you start to zoom back (below about 120mm) you might as well take it off, and you are losing a stop of light as well as the 70-200 becomes a 5.6 through it's whole range.
I dislike having to change lenses at the best of times and changing the convertor on and off is not an option for me, so I tend to overlook shots that require 70-150mm when I have the convertor fitted.
David