Leica/Leitz lenses are not really "better" but they do have a different look to them. I use Leicas soley for black and white and I like using Leica rangefinders better than using any SLR I've ever owned (Nikon and Canon EOS). Lens quality is not really a concern since everybody makes good lenses. The Leica excells at allowing you to shoot at slower speeds and take quick candid photos. It's not as versatile as an SLR, however, and I would suggest you keep your Nikon to be able to use long lenses, do close-ups, etc.
By the way, Leicas focus in the opposite direction from Nikons. If you are used to the way Nikons handle, it can be frustrating when you try to use a Leica.
Used Leica equipment can be found at reasonably reasonable prices. You should be able to get a really fine body and 50mm Summicron for less than $2000 if you shop around.