I had the 120-300 sigma 2.8 it was a really nice lens, heavy and you needed a mono pod or such to hold it for a long time. My friend has the 120-400 with OS it not as fast but I gave it a try and for the money it's not bad, A little soft but your not spending the extra 2 to 4K on a 2.8 pro lens. Sigma new 70-200 2.8 is a killer buy it fast and rated almost the same as the nikon. would give you a 300mm in a DX body.
I own several Sigma lenses along with numerous Nikon Tamron and Tokina lenses and have nothing but praise for the one's I currently own.
That certainly produce the goods and represent excellent value for money if you get a good one.
(there quality control can be a little suspect at times)
Are you looking for a fixed 300mm or a zoom? For my money, you can't go wrong with the Nikkor 70-300 VR which is a little over US$500. My brother has the 70-300G (no VR) which is pretty nice as well. It is pretty inexpensive at around $150. We're close enough if you want to try the 70-300 VR out. . . .
I have the Sigma 120-400mm OS HSM lens that is excellent for wildlife, birds in flight, and landscape. It is large and heavy, needs a tripod or other support, and is a bear to lug around. Bought it at a real good price as an open box item (looked brand new and the glass was clean).