Now that iDrive usability is improved, I'd definitely go with the built-in Nav. It will increase resale value, and look better in the car than an aftermarket unit stuck to the windshield.
Additionally, if you opt for the HUD, then the HUD display will be linked to the Nav system so the directions are visible right in the HUD display, like so:
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2016 X1 M Sport xDrive28i ED 12/2015, re-delivery 2/2016 -- Estoril Blue, Black SensaTec, Sport, Premium, Cold Weather, Tech, Driver Assistance
2006 650i Coupe ED 11/2005, re-delivered 2/2006 -- Jet Black, Dakota leather, 6-speed manual, Sport, Premium, Cold Weather, Tech/HUD
