I'm looking to get a new lens. Ideally, I would love to get the Nikon 70-200 VR 2.8. Price is the only issue, but it's a big one. I'm considering a cheaper alternative, either the Sigma 70-200 2.8 or the Tamron 70-200 2.8. Does anybody have any input on what might be my best decision? Or has anyone had any experience with either the Sigma or the Tamron?