12 year old Vitiate was able to strip a sith lord of his sanity. 8 year old (?) Vitiate could snap his father's neck and slowly torture his mother to death. From these two data points, we can sort of interpolate back to newborn-Vitiate if we curve-fit it relative to how other skills are known to develop over time, and then scale to Luke's expected latent power vs. Vitiate's own. This might make an overprediction between the dark side gains you power more quickly, but ah well.
From these calculations I declare ROTS Luke to be the winner.