I think Johnson was quicker on his feet and his technique was a bit more polished. I also don't think Conteh fully developed his skillset until after he had already injured his hand (and possibly only because the injury forced him to). I'll take Johnson on points.
Conteh is really overrated IMO. He loses a clear decision, because Johnson was much better. Johnson would win the battle of the jab, and that'd be where the fight is won. Sure, Conteh probably was a bit better of a defensive whizz, but so was Pastrano, and Johnson just got on with his business there. I suppose in a best case scenario for Conteh is that he makes it a Larry Holmes-Carl Williams type situation. But if Ezzard Charles can't, then neither can Conteh.