I think shutterman has a very good point when he says whatever you pick you'll do fine, at the end of the day a camera in the hand takes pictures a camera in the shop doesn't
However, if you are going for a D80 or a D200 S/H definately offers the best value - most will have less that 10K on the clock and you are unlikely to loose much in value over the next year on either. Plus 10K on the D200 is barely run in.
Whatever you choose always invest in good glass - it will last much longer than any digital body you buy. My advice always buy Prosumer or Pro spec lenses, IMO they'll make more of an impact on your photos than the latest DSLR with bells on.
To help you I find lenses generally fall in to 4 brackets
Budget - cheapest lenses available, usually plastic mounts and bundled with kits, they can be soft at all but optimum apertures and exhibit distortion and chromatic abrasion e.g. 18-55mm and 55-200mm in both Nikon & Canon not usually avilable seperately sometimes as little as £40 difference between a body only and a body + budget kit lens. The general rule of thumb for budget lenses is that Nikon produces the better quality budget lenses (but they cost more).
Consumer - Better quality in both build quality (metal mounts) and image quality. Produce sharper pictures than budget but may still be limiited by distortion and chromatic abrasion issues (which can be corrected in software) such as Nikon 18-70mm / Canon 17-85 IS / Nikon 16-85 VR / Nikon 70-300VR usually £250 - £400
Pro-sumer - Top notch glass with either older or slower Pro designs generally very sharp and have few issues with distortion, chromatic abrasion e.g. Canon 70-200 f4 L, Nikon 80-200 f2.8 usually £600 - £1k new but can be found S/H quite reasonably as semi-pros upgrade to Pro glass as they can afford it.
Pro - Top notch glass designed to match the capabilities of modern digital cameras, pin sharp with minimal distortions e.g. Nikon 70-200 f2.8 VR, Canon 70-200 f2.8 IS but this comes at a cost - usually £1k+ new and retain a lot of value on S/H market.
Final word - if you've tried them for size and a set on a new camera rather than S/H toss a coin and if you're not happy with the result you know you really want the other one - just go buy.
Hope that helps