Canon's will give the best optical quality ('best' being a relative term, here). Of the two, the 1.4x is optically better, significantly so. Be aware that the 100-400mm will not autofocus with either extender on anything other than a 1-series body (in which case it will AF with the 1.4x but not the 2x), and the 70-200 f/4 will AF with the 1.4x on all bodies, but with the 2x only on 1-series bodies.