The real advantage to the Nikon system (in addition to the fact that you already own some Nikkors - the 105 f/2.5 is supposed to be very nice) is the fact that it is still producing lenses, and cameras for the same mount system. Neither Oly nor Pentax (nor Canon, although you did not mention them) are backward compatible. Nikon is getting less so (G lenses for example) but for the most part, new lenses work on older cameras. Nikon also still sells two current film SLR cameras, one of which is manual focus! (The FM-10. Nikon doesn't make it, Cosina does, but it is a Nikon design, and says Nikon on it.)
If you are mostly a 50mm shooter, and a bokey fan, you will probably want to look at the OM system. There are lots of cameras in the used stream, and lenses are also abundant. I am still working on a request to compare and contrast the overall look of Nikkor 50mm with Zuiko 50mm lenses. The Zuikos look better to me. Although older Zuiko lenses can be put on newer Olympus cameras with adapters, newer lenses do not fit on OM cameras.