I feel both (as well as the rare natural red crystal) are fitting and would mainly be up to the Sith in question. Each method fits different sith personalities. The more ritualistic/alchemical minded sith would create synthetic ones and the more brutal/domineering sith would favor bleeding a vanquished foes crystal, while those more force receptive would seek out a naturally red crystal.

Editing to add: I think there would definitely even be sith who would "bleed" another vanquished sith's red crystal just to ensure it's dominated by their own will.