Tubes have no optics, just air and electrical contacts, Canon charges a lot for the air in theirs.
But really cheap ones can break at the mount (many are plastic or weak metal). Kenko is good, if you can't get them, I'd say Canon.
Personally, I went with a Canon 25mm tube - I'm not going to risk the mount failing, because my main use for it is to reduce the MFD of my 600 II.
+1 on the Velbon Super Mag Slider, best of the 'budget' options. For best quality, Really Right Stuff is the one.