Neither the Canon 18-200
nor the Nikon 18-200
is particularly good. They both suffer from the same shortcomings inherent to all superzoom lenses. When you try to make a lens do to much, it doesn't do anything well. They have more rectilinear distortion and chromatic aberration, and are not very sharp.
If you're concerned about image quality, you'd do better by getting multiple lenses of less ambitious zoom ranges.
If you want a 'jack of all trades and master of none' lens, then the available lenses shouldn't be a reason to select one camera body over the other.
What matters most, I believe, is how the camera feels to you. If you can find the controls and commands when you need them, you'll be more successful and happier with your choice. And considering that you have a Nikon D5000 and will keep it, it will probably be easier for you to switch back and forth between two Nikon bodies than between a Nikon and a Canon.