I have a Nikon 300 f4 and it is truly a great lens for the money. I had a Tamron 200-500 and there is just no comparison between both lenses. The Tamron was good, don't get me wrong I took some good shots with it - have a look at
http://www.mejhogan.com - both the Fulmar and Glendalough - but its sweet spot was not the easiest to find and I would presume the Sigma will be the same. The Nikon is always crisp and clean even with a 1.4x converter on it. I'd go for the 300.