Mike Hitchen , Dec 22, 2010; 04:38 p.m.
Reviews do not seem to show significant difference betwen the two. The Canon has the better build, the Tamron the additional reach.
My preference wouldbe for the Tamron. Although I think either would do you well, the additional 10mm range of the Tamron would make it far more versatile. I have the Canon 17-55 and bought it because although the 17-40 was excellent I found myself wanting a little more from a lens I hoped to have on my camera around town.
You say you want to move to a 'Mark II'. Do you mean a 5DMkII? The Tamron will not fit on 35mm bodies and the 17-40 would be yor better bet. Normally I would say buy lenses for the camera you have now, not for one you might get because you may find yourself frustrated by limitations (of whatever sort) in the meantime. So unless getting the 5D is a definite probability go for the Tamron.
Just to confuse matters, the new Sigma 17-70 is getting very good reviews.