My $0.42,
For dual-purpose (track and street), use magnetics. I really don't want to draw any more attention from the poh-poh when driving a Porsche on the street.
But while you are at it, you might as well put all those decals that the import guys seem to equate with driving ability and turning a grocery-getting Civic into a bona-fide racecar, like this:
For track only cars -- they don't belong on the street, so the numbers can stay.
Edit: Just noticed on that car -- the second decal from the top - TRD - stands for Toyota Racing Development. Car decal fail!
-Z