Hi Jason,
I own the Nikon 70-300mm/4-5.6 D ED AF lens. I really like it. I tried the Sigma first, but the quality and speed of the Nikon is what I preferred. I take a lot of sports photos with it so the speed was very important to me and I've had no complaints. The only thing I'd prefer is the 2.8 lens for lower lighting conditions, but can't afford that one. I'm waiting for the day they create an affordable one and I'll be first in line.
Hope this helps...have fun shopping.