I think both are good for astro work. Canon has an IR filter which makes reflection nebulas less 'red', I don't know about Nikon. But I have a Canon 40d which makes excellent astrophotos.
Canon has a larger bayonet mount which enables
adapters from ebay to fit Nikon lenses to the bodies. Old lenses fit as well, but no autofocus. Similar adapters exist for all other mounts (including your existing Olympus lenses) except Canon FD.
The 60d is even better in noise reduction you can take pictures @ 3200 ASA which I sometimes do with the 40d.
Look on the internet for noise comparisons between these cameras.
Of course fullframe is better yet (Canon EOS5Mk2 or Nikon D700) but prices are > $2000 body only.