While the picture quality is equally good in both Nikon and Canon, the difference lies in the way the two cameras feel, the way the menus are setup, and where the buttons/controls are placed. In very few cases the lens availability is a factor, but price wise, the two brands are very similar.
If you are going to shoot weddings, Nikon controls/buttons are much more intuitive. The D300s or the D700 will work perfect. If you are a Canon shooter, the 7D is all you need. The more expensive models (Nikon D3s or the Canon 1Ds series) would work great too.
If you are going to shoot a lot of studio work with only hot shoe flashes via infrared, any Nikon model from D90 up will work great because they have a built in IR transmitter. The Canon 7D is the only Canon that does that. If you are going to use other remote triggers, any Canon or Nikon will do the job. Under ideal light conditions (studio), all cameras perform great, so it doesn't really matter if you have a Canon XS or a Canon 7D - they will both take similar photos.
If you are need a dedicated camera for landscape work alone and under $3,000, a full frame camera such as the Canon 5D Mark II is your best choice. The Nikon D700 is a great alternative if you are a Nikon shooter. If you are on a generous budget, the D3x and Canon 1Ds Mark III are your best cameras on the market.
If you are going to shoot sports, the Canon 7D is an amazing camera for such things. So is the Nikon D300s and of course, the more expensive models - Nikon D3s and the Canon 1Ds Mark III OR IV.
If you are going to shoot video, the Canon 7D is exceptionally good, and so is the 5D Mark II. I would recommend the 7D.
If you just need a good camera for everything, I recommend Nikon over Canon, simply because of the way Nikon controls and menus are designed. Any of the cameras listed below will work great - it is very important that you buy the one you can afford.
If you haven't noticed yet, all the recommendations above have to do with relatively high end cameras (Nikon D90, D300s, Canon 7D, 5D Mark II, etc.), which start at $1,000 for the body alone; needless to say, they need equally good (expensive) lenses to take full advantage of their features. If your budget is under $800 any of the recommended Intro & Amateur cameras will work very well. Buy the one you can afford.
If you have a question that I didn't address, send me an email at mail@titusbartos.com. If you are still not sure what to buy, please include your budget and what you plan on doing with your equipment. I should be able to reply within a day at the most.