I've currently got both the Canon 1.4 and a Kenko 300 Pro 1.4X, I've actually stacked them on my 400 f5.6L because the cam still AF's (2nd TC doesn't report) and the IQ is better then a 2X by itself. The advantage of the Canon over the Kenko is slightly better color and better build quality and it's white so it matches your L teles. Canon disadvantages over the Kenko: more expensive, larger-heavier, and won't work with all lens because the canon has a protruding rear flange.
Sharpness is a draw AFAIC based on a bunch of tests I did with both on a 300 f2.8L IS. Both TC's report to the body so as Walt/Scott pointed out you lose 1 stop automatically. Tamron makes a cheaper version that actually doesn't report so your 70-200 f2.8 will still be an f2.8 (at the expense of some sharpness as it's just not built as good). If you are planning on using it a lot and want to squeeze every drop of quality possible get the Canon, if not save some money and get the Kenko because it's 98% of the Canon IQ wise.
A couple of examples with the Canon 1.4: the bird with it on my 70-200 f4 IS and the the DB shot with it on the 400 both shot handheld.