I would go with the Canon 1.4x - stay with Mfgr and Mfgr ... the lens is build by Canon for Canon ... and if you go with Canon lenses later too, it fits. The savings is not great enough to sacrifice the quality. You bought a Canon camera for a reason. Earlier in my career, every time I tried to save a few bucks, it always cost me double, buying the less expensive version, then selling it for a loss, and buying the more expensive version.
Not trying to 'bad-rap' other manufacturers here specifically - but Canon makes a good product. I know, I know, money talks savings, but quality talks too. You'll never get a chance to shoot that image again, and you have it forever if you ever decide to use those images later for other reasons (like sales?)