While it is possible that both do well in 2018 I think Onion has more room for growth in 2018 and think they will do better. Although they aren't 100% TOR secure yet I think with the next update they are planning to integrate that and with the incoming technology your coins with always be able to be mixed, have ring signatures, and zk proof. Big times ahead for Onion I mean if a shitcoin like Verge can moon, literally anything can.