G'day Mick,
I reckon for $959 delivered the Nikon is a bargain.
http://www.digitalcity.net.au/store/...idProduct=1923
The Canon is a more popular choice for astrophotography. I'm yet to see any astro photos taken with a D50 so there's no true comparison there, but I doubt there's a big difference between the 2 cameras, just that the Nikon is a few hundred $'s cheaper.
The Nikon D50 is said to have better noise performance at ISO 1600 than a Canon 5D which is apparently the benchmark for low noise at high ISO. This is according to Dec 2005 Popular Photography magazine. It sounds too good to be true (and possibly is?), but we can still assume the D50 has pretty low noise. I've used neither one of them so I can't offer you any first hand experience.
If you're on a strict budget the Nikon is a clear choice to me, but if you want to spend an extra $300-$400 the Canon is also a very good camera. I'm sure either one will take pictures that you'll be happy with.