I have both as 6* and Ghost has been clearing more content for me than g2099. Rank the one that you use more.
Do you have ghost leveled up and or awakened as a 5 star?
If you have a high sig 5* Ghost do G2099 because a higher sig 5* Ghost has higher DPS than an unawakened R3 6*, while G2099’s Ability doesn’t make much difference.