Snap on for me but if you after the IPhone 6 the snap on option is useless as it consist of a bracket holding the phone and having to plug the cable manually due to the size of the phone.
One option you should check is the new 7 series wireless charging case and snap-in adapter if they fit the 5, looks very cool and tidy.
http://f30.bimmerpost.com/forums/sho....php?t=1166484