Just to add my 2c I went for the Sigma 18-50mm F2.8 DC HSM, Got it price matched in OZ from the CR Kennedy price matching for $399.00 brand new with full Aussie Sigma Warranty and service. Its a DC lens, so it designed for the APS-c sensor, perfect fit for your lenght fill. Have a look here - http://www.sigmaphoto.com/shop/standard-zoom-lenses . It has HSM (High Speed Motor) same as USM (Canons terms for Ultra Sonic Motor) (CR Kennedy.com.au type in price match in search box, they are backing their Aussie re-sellers to match Grey Market imports, so get a Grey price from say DWI and take it to your quality local camera store, they'll match it)

Have fun. P.S. I compaired the above lenses you've mentioned and for the price the IQ of the Sigma is just as good as the 17-55 mm Canon, at 1/4 the price. Between that and my 70-200mm L they cover all aspects nicely. The 24-70mm you must x 1.6 to get accurate focal length for crop sensor 38-112mm approx.