Upload & Sell: Off
Funny, I think of Kenko as 'off-brand'. I've seen Kenko sets for $135, which is a lot cheaper than Canon.
There are no optics in a tube of course (just air), so the image quality shouldn't be any different, unless the connection is 'wobbly' and it won't keep the lens aligned properly with the sensor
The main difference is in
1. build quality
2. electronic connections
For Canon I think the electronic connections are essential; without them you can't control the aperture and you'll always be shooting wide open (well, I guess you can hold the DOF preview button before ripping the lens off the camera, but it's a lot of jacking around and it's hard to see/focus through a stopped down lens)