Blue tooth or WiFi? My bet is on WiFi for the main reason of Apple perfecting the application car audio manufacturers have been trying to do within the last couple years. There have been various attempts of wirelessly synching car audio with the home audio (computer) database. MacMini car installations can now do this seamlessly, but a Mini vehicle installation is not exactly seamless.
DaimlerChyrsler/Jeep have just signed with Apple to offer an iPod interface in their future vehicles. Various other manufacturers have been offering this upgrade as well (BMW even did an entire ad campaign). The iPod is integrated into the car audio system, neatly tucked away in the glovebox; will I want to pop my iPod in and out daily for updates? NO!
Drive up to your house/condo/apartment/hut and your home computer system will detect your iPod in the WiFi cloud. Barring any Apple plans to remove the battery from future iPods, this function will even work while the vehicle is turned off.
The sweetness factor is you will be able to update "325xi" through your iTunes via a WiFi iPod. I'll keep the Bluetooth for my cell phone, thanks.