I haven't been impressed with the Sigma or Tamron lenses I have seen. In lenses, you generally get what you pay for and if you are going to regularly use the lens, buy the Nikkor. For a rarely used lens such as a fisheye, you can get by with cheaper.
If you are going to buy a high end body, you negate the benefits of it with lower quality lenses.
I would generally agree with the above, however I use a sigma 70-300 OS and I find it to be very good, I suppose you get what you pay for, if you go with the higher end more expensive lenses then the result will show in your pictures
I had a Tamron version of this lens, what a horrible let down, soft pictures, no IS/OS and slow AF, generally not that great (traded it in for the sigma)
Haven't tried the Nikkor, heard it is very good though
In my conclusion, try each one before you buy, work out your budget and try to get the "better one" if you can
I generally don't skimp on kit, If I can't afford "the best" then I'll wait until I can