Both are very good. The 350D loses the B&W LCD from the top, which can be a pain for a quick galnce at certain settings.
Personally, I find Nikon images have far more 'punch' straight off the camera. I do not believe the bog-standard Canon lenses are that good, giving a milky quality and lack of biting sharpness. The ordinary Nikon lenses are better.
However, the 'L Series' Canon lenses are as good as anything Nikon can produce - sharp, detailed rich. I've seen L-series Lenses on cheap 300Ds produce result you'd swear were 'high end'. They are also a fair bit cheaper than the pro-level Nikon lenses.
I want to upgrade my D70s and like the look of the Canon 5D, which is slightly more 'pro' than the D200 and produces fabulous result with a good lens. However, a switch to Canon would mean ditching two half-decent lenses and starting again so I'll probably go for the well-reviewed D200.
As another poster said, when you go for one company or another you're pretty much locked in unless you want a lot of expense.
Also, don't waste money buying lame lenses you'll only want to upgrade later. Buy good glass, which you'll be using for years, and don't get too hung up on the camera back. I'm pretty sure a D50 with Nikon's 17-55 wide angle would produce far more wow-factor than a D200 with Nikon's cheapo 18-55.
And I'd go so far as to call the 18-55 plastic Canon lens you find in kits with the 350D and 30D 'a bit crap'.