Canon EF-S 17-55mm VS Sigma 17-50mm VS Tamron 17-50mm?

psycho, I've enjoyed reading the "discussion" regarding which lens you should get. LOL!  Lots of great advice.  I am just a beginner and have begun work on accumulating lens's.  An important question I had to answer for myself is what do I intend to do with said lens and what do I want the lens to do?  In your situation, will carrying multiple lens's be an option?  Or do you want one lens to cover a "majority" of the shots you encounter?  Toting 3 prime's seems like it would be a pain.

I recently purchased a walk around lens for my T3i.  I too was looking at the same line up of lens's you referenced.  After spending countless hours reading reviews, I went into my local shop and tried them all out.  At my beginner level, I couldn't tell much difference.  The AF speed - maybe.  Build quality, weight, function, appearance - tie in my opinion.  Then came the quality of image captured.  The sales guy kept trying to point out to me center sharpness, edge softness, etc... They all looked great to my novice eye.  The biggest difference in one over the others was the price.  I went with the Tamron non-VC and feel great about my decision.

However, I've run into recent situations where I wish I had more zoom with me.  I only take the one lens when I go walking around downtown NOLA.  The 5mm extra on the Canon isn't a reason to choose that one, but have you thought about the 24-105L?  It's only f4 but it's a versatile range for walking around.  At $750 you will have plenty extra to purchase the Canon 430exii (which I just purchased :)) and a possible third item - a ultra wide prime to fill your low end needs.

Good luck on your decision.

Adam