Both types are good, both have pros and conts and at the end depends on what the player likes.
I you like DPS (aka APS) and crit well then go demon. Demon version has longer stuns, more attack speed, and demon tidal is pretty good cause it has a chance to activate focused mind with it (means that there is a chance to receive no damage) but demon tidal is only 50% chance to avoid effects.
My sin is sage (sage tidal is OP with 67% = god mode against status for 1 minute), has 81% base crit with DAMAGE PER HIT set. My set is overrated and most of the sages have around 60% crit. If you like to change between APS and DPH go sage. Sage attack skills have more damage then demon ones. But that doesnt mean demon sins cant dph, no. I have seen demons DPHing pretty well, and the demon tidal helps a lot since is DPH and the damage is higher.
SUMMARY : Pick the spark color you more like.
To be a good sin, depends on how skilled you are not on which version you pick.
