There is a thread about mounting outside a car: http://forum.mapillary.com/t/experience-with-mounting-a-camera-outside-a-car
In short I would say:
Suction cup: Do go too cheap, mount on a clean surface and take great care in doing it correctly. They hold up good when done right, but I have taken great care and had them fall off quickly.
Magnets: I plan on getting a ram mount. Again, don’t go cheap. Should not fail as catastrophically as suction cups.