As someone already has said, I would try to work out a deal with a dealer to buy the one with the Navi.
Lets think about this for a bit. You forget your portable GPS in a car. The next day someone breaks in it. It would not be worth to claim it and what not. That money, hassle + tiMe wasted would go for an extra mile to get a 335i with the navi. I would think about it. Trust me, I have seen bunch of those scenerios all over the GTA.