I shoot now with a Nikkor 55-200 f/4-5.6 lens. It's slow and terrible in low light. I'm looking to upgrade and was looking into either [ame="http://www.amazon.com/Sigma-70-200mm-Macro-Digital-Cameras/dp/B001044RIQ/ref=pd_bbs_sr_2?ie=UTF8&s=electronics&qid=1237679670&sr=8-2"]Sigma 70-200[/ame] or [ame="http://www.amazon.com/Nikon-70-200mm-Nikkor-Digital-Cameras/dp/B00009MDBQ/ref=wl_it_dp?ie=UTF8&coliid=I2BPMWU3XKOAII&colid=11LM30JRK1WUQ"]Nikon 70-200[/ame]. Obviously the biggest difference is price but I was wondering how much I'd sacrifice by going with the Sigma over Nikon. I've read good reviews for both, though I think the Sigma is heavier...not something I'm going to worry about at this point.