6* key would surely be closer to correct timing. There is no way to know for sure since the keys are not made exact and no two flywheels and crankshafts have the keyways cut exactly the same.
6* would sure be closer though.
FWIW, I generally don't use a key at all. Simply lap the flywheel to the taper on the crank with a fine valve lapping compound, clean, set the piston in the hole BTDC where you want it (30* in your instance), align the trailing edge of the magnet with the right hand edge of the pick up module, and torque to 50-60 foot pounds. Done deal and you know exactly where it is.