Canon 70-300L IS USM Vs Canon 70-200L F4 IS

70-300L IS USM all the way. No regrets. So sharp at 300mm its amazing. Takes the Kenko Pro 1.4x too with little effect on IQ. However I use it without the TC most of the time. Never had an issue with CA. I also have the 17-55mm but would replace the 70-300L first every time.

Good luck with your choice.
Gelbvieh