Either camera will do you well - they're both excellent. I think the Canon has better high iso performance (less noise) and is a tad better built. Check them out at the store and see which feels best.
When deciding on a camera, while the body is somewhat important, you are really buying into a lens system. The camera will be outdated every couple of years but the lenses can last you a lot longer if you look after them. I hear Nikon has superior but more pricey glass (I shoot Canon).
Two things to consider if you buy the 30D
1) Rumor is the 40D is around the corner, but I have seen no evidence whatsoever to substantiate that. The PMA show is in March and sometimes coincides with product announcements.
2) You can get a rebate if you buy before 13th Jan here is the link.