The Kenko Extension Tube Set is very popular because it is a good value and has high functionality.
The Kenko Extension Tube Set's is functionally similar to the
Canon 25mm Extension Tube.
But with three sizes included in the set (though 25mm is not one of them), the Kenko Extension Tube Set offers more flexibility for similar money.
I searched hard to find a physical difference between the Kenko Extension Tube Set and the
Canon 12mm Extension Tube.
What I found is that they are far more similar than they are different.
Probably the only significant downside to the Kenko Extension Tube Set is that it (including the "DG" version) is
not Canon EF-S Lens compatible.
Note: I am hearing that the current Kenko Extension Tube version is now EF-S lens compatible.
The Canon 12mm and the Kenko 12mm extension tubes are identical in weight (2.2 oz)
- it is unlikely the one is much stronger than the other as they both feature metal mounts.
The other Canon advantages include nicer caps, an included lens pouch and a very slightly more attractive textured finish.
Of course the Canon tubes have Canon's name on the side.
The Canon tubes have a white mark for EF-S lens alignment which is also missing on the non-EF-S compatible Kenko tubes.

Pictured above is the Canon 12mm Extension Tube with the included pouch (left and center) and the Kenko Extension Tube Set.

Can you tell which of the above tubes is Canon and which is Kenko?
The brand differences are obvious in the picture below, but their similarity also shows.

Recommended Shopping Resources
(Using these links to make your purchase supports this site)
Kenko Extension Tube Set at B&H Photo
Kenko Extension Tube Set at Amazon.com
Kenko Extension Tube Set at Adorama
Find the best price on the Kenko Extension Tube Set
Rent the Kenko Extension Tube Set
(Using these links supports this site)
Kenko Extension Tube Set at LensRentals.com
* If out of stock, ask the rental company if they can accommodate your need.