I shot with a Leica m3 and Nikon FM2 and N90s for years. I'm an old film leftover until i got my K10D recently.
I don't think you can really look at it as D3 vs. Leica.
I have always felt having a slr(s) and rangefinders and complementary. They both have different uses for different subject matter. If you need a quiet camera that is inconspicuous and you are happy with leica's prime lenses or vari lenses by all means it is the camera for you. If you need zoom lenses, telephoto lenses or do macro work an slr is going to be the best choice.
Since you have a K10D, are you happy with that being your slr? Does the Nikon system offer something that you need? If you are searching for the highest IQ the D3 is going to offer it.
I would choose the Leica, keep my K10D and use them both.
If Pentax would make a rangefinder for use with their limited lenses, life would be a lot easier