No question about it, the 5d Mk II is capable of better image quality. If you have money to burn, go for it; I'm sure Canon can use the money and the rest of us could enjoy another barely used D90 on the market. But if you can't get good images out of the D90, you will have a lot of learning to do.
While I'm a Nikon shooter and think they make some great cameras, I also think that Nikon does a fairly poor job with its default settings. In the recent models, I'd start by turning down the brightness setting in whatever picture control you use, as the default tone curve tends to brighten mid tones so much that the images don't look very good to me. I initially thought my D700 needed negative exposure compensation all the time until I dialed down the default brightness setting. Experiment with your settings and see what you can get out of that camera. There are certainly thousands of marvelous images on line from the D90 to inspire you. If you don't want to take the time to mess with the various settings, then a DSLR might not be the best camera for you, and in particular the higher up the line you go, the more the manufacturer appears to expect you to know how to dial in what you want. There are no "scene modes" on my D700, for example.
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Good shooting,
GR
North Carolina