Have you considered the PDA based navigation options ?

That way you can use them in any car and you don't have to worry about finding the car with the right nav system.

It is also cheaper, in the UK at least. A builtin nav system is typically a £1000-£1500 option in the UK. A decent PDA, memory card, decent mounting kit, GPS and something like TomTom Navigator adds up to about £750.

The downside is that it is another thing to take with you when you leave the car (I leave the GPS in the glove box and the mounting kit, I only take the PDA with me).

The other good thing to this approach is that you can upgrade the software as it improves, not an option on a built in system.