I would normally agree with go with Canon. However, I had both the Canon 1.4x and 2X and neither one were worth a crap. Aside from being way over priced, mine did not give me any images that were worth of anything but plain documentation images. I used them on a D-60, 30D and a 1D Classic, with a 100-400, a 400/5.6 and other Canon lenses I owned at the time. Plus, a 600mm equivalent telescope with an adapter for the EOS lens mount. When I got the Bigma, neither TC would fit it. I traded them for a new set of Sigma 1.4x and 2x TC's, which did fit, and gave a decided increase in IQ. I still have the Sigma 1.4x and use it quite often.
To be fair, I have seen many other Canon TC's used here and the images posted from them which look quite good. And most of those images came from the 1D MkII's or newer bodies. I just think the Canon TC's were made for the Pro bodies. Just my opinion. But you can't really go wrong with the Sigma's either, and they are cheaper.