Upgrading 70-300 L to 70-200 2.8 II IS + TC?

[Chris102]

I use a Canon 5D Mark II and 7D, and my telephoto lens is the 70-300L. However I'm trying to figure out whether it'd be a good idea to ditch the 70-300 for the 70-200 and a teleconverter.

I recently rented a 70-200 2.8 II IS and was very impressed by the quality of the images--I thought they were a marked improvement over the 70-300. I know I would also benefit from the 70-200's 2.8 constant aperture, as the 4/5.6 was very limiting on the 70-300.

For those who have used the 70-300 and newest 70-200 2.8 with a teleconverter: is the quality of the 70-200 + 2X or 1.4X TC at least on par with the 70-300 alone? I can't afford to keep both, so if I were to get the 70-200 I'd have to first sell my 70-300. I should mention that the lens would be used for sports and aviation photography.

Any advice would be greatly appreciated!