They are (reportedly) about the same in the center, and the Canon is better at the edges of the frame (even on a crop sensor, more so on a FF). If cost is the issue, consider a used Canon extender. I bought a used, like new 1.4x MkII last week ($180 on my local Craigslist). Even if you're normally reluctant to buy used, the nice thing about extenders is that there are no motorized parts, just non-moveable optical elementsand solid-state electronics.