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The Kenko seems to sound like the one to go with, and from what I've heard as well it performs very well.
The 120-300 from what I've read is around the same sharpness as the 300 2.8 prime, but the downside is that I've read about a lot of cases of the 120-300 not actually reaching 300mm, and resulting more around 260mm? Being able to zoom is a plus, but in most cases (i.e. field sports) I'd be using the 300 on my 1d3 (with or w/o the 1.4x depending on light) and the 70-200 on my 5d2, so it's not as important.
In the end, both seem like great options to the very costly Canon 300 2.8. On the flipside, I shoot for a newspaper so razor sharpness isn't necessary, just as long as it's relatively sharp. Though, I've used the 400mm 2.8 on some assignments and the quality is amazing ($10,000 of amazing, sadly).