Narrower zoom range, hence the price difference; also the slight difference in max aperture. It might be the way to go though for that reach and the price point.
Technically, you can grab the Sigma 70-200mm f/2.8 for about $800 and then pick up a 2x TC for another $300 to bring it to about $1100. That'll get you to the 400mm f/5.6 area and will still allow you to autofocus. Not as far out as say the 500mm and I'm not sure how well the 2x TC performs. The newer Nikon 2x TC (III) does a decent job and I've seen people use it on their 300mm and 400mm f/2.8.
I'm guessing you'll be better off w/one of those lenses without the TC. Expect to stop down any of those lenses to about f/8. Also, keep in mind that at 500mm or so, you're looking for at least 1/500 as far as shutter speed. Not sure how much light you might have to shoot with, but that might be an issue, so be prepared to push that ISO.
Long lenses are something that I never really invested in due to the high costs and the limited uses. When I was still shooting birds, I got by with the 300mm f/4 AFS coupled with the various TC, which got me out to the 420mm-600mm range. Using 2x TC was always iffy since the details didn't hold too well.
300mm f/4 +1.4x TC (420mm f/5.6)
300mm f/4 + 1.7x TC (510mm at f/8)
300mm f/4 + 2x TC (600mm at f/11)