I have a new D50 (two, actually), three Nikkor lenses, and a Sigma. I believe the bottom line, as usual, is that you get what you pay for. Nikon and Sigma both have good lenses, and some not so good. Follow the dollars. I have read some good things about the Sigma 70-300 Apo (red ring) and, if I had not been able to get the 70-300VR Nikkor, I'm pretty sure I'd have ended up with the Sigma. Beyond that, I'm not too sure about the quality of the lenses you mention.
Something else to keep in mind is the 1.5X crop factor on the two lenses you are looking at. Nikon uses APS sized sensor, so the focal length X 1.5 is the "35mm equivalent" in terms of field of view. Ergo, the 28-70 becomes 42-105mm and the 70-300 becomes 105-450mm. You gain on the telephoto end, but lose significantly in wide angle.
How does the "package" price stack up to buying a D50 body, a Sigma 17-70mm and the Sigma 70-300mm separately?
$450 D50 body (B&H)
$325 Sigma 17-70 (Sigma4Less)
$180 Sigma 70-300 Apo (Sigma4Less)
$955 Total
~YEKCIM