which extension tube?

I try to limit my travel kit to my Canon 40D and 17-55/2.8. I'd like to bring along an extension tube for occasional close-ups, too. For that camera/lens combo, which makes more sense: the 12mm or the 25 mm?

12mm, unless you are into true macro range.

12mm tube on 50mm FL = about 0.25x lifesize (20mm object is 5mm on 15mm frame height)
12mm tube on 31mm FL (APS-C 'normal' FL) = about 0.4x lifesize

A set of Kenko tubes costs about the same as a Canon 25mm tube and then you have three to choose from.Also I believe a tube will allow you to mount your TC to some of the lenses it will not fit on now
for added "magnification"

The newer Kenko tubes are EF-S compatible. Adorama didn't specify which version it was when I boutht at the end of last year, but EF-S OK.

I have the Kenko set and recommend it too. 3 rings really help as different lenses and situations might require different amounts of extention.
Build quality is good enough for anything you might throw at them.

I use the Canon 12mm a lot on my 24-70, I use the 25mm on long lenses like the 70-200 and 100-400. Rarely have I ever used the 25 on anything shorter than 85mm.