Chaos ring is much better, you can cube it 10 times a week for 6 months. If you're in reboot, roll it until you get +5 att/matt and a decent +stat depending on class. In non reboot, do it until it gets 18% stat or more.
@cris1000: but the Chaos ring is insane expensive. Its 3500 Ark coins (or is it 3000 ?) And the Synergie is 1000 Ark coins. Which is as many as i have right now, lol. Would it be even possible to get 3500 |Ark coins before this event has ended ?