The IQ penalty is probably my primary consideration but I've read varying testimonies regarding the 1.4 versus the 2x.
There was an article on line once (gone now) that compared the Canon 1.4x III, 2x III, and Kenko's. Canon's 1.4x III did not have any better IQ than Kenko, but Canon 2x III was clearly better than third party 2x TC.
I have the Kenko 1.4x, Kenko 2x, and Canon 2x III, and I would say Canon 2x clearly has better IQ than Kenko 2x, and if stopped down to f/8 on 70-200 f/2.8 II (with adequate shutter speed), the Canon 2x comes very close to the 1.4x in IQ.
So I would recommend getting Canon 2x III as the relative best buy, but if you're worried about 2x IQ loss anyway, also buy the Kenko 1.4x which is excellent and cheaper.