I'd like to purchase a fast zoom lens to go with a 20D, mostly to photography my children with. I'm looking at three f2.8 zooms: the Canon 24-70L, the Sigma 28-70 EX DG Macro, and the Tamron 28-75 XR Di LD. Does anybody know of any professional comparisons or have any personal experience with two or more of the lenses? I'd love to save the money and get the Tamron or the Sigma, but I'd be willing to splurge on the L if it's worth the extra cash. I'm the king of indecision -- help!