How does the Sigma 70-200 F2.8 HSM compare to the AF-D version of the 80-200 F2.8? The more I think about it, the more I realize that if I'm going to spend $850-ish on a lens, I'm going to want a warranty with it incase something happens. eBay doesn't quite offer warranties on used equipment. Does the Sigma's HSM motor bring it into AF-S territory? (Focus speed wise) It seems like the Sigma is a far better deal, but I wouldn't want to sacrifice quality just to get a little quicker focus.