The best choice will almost always come down to your talent choices. If you are talenting into abilities that enhance Emerald Blossom such as Fluttering Seedlings, you will want to cast Emerald Blossom exclusively. If you are talented into Time Lord, you will primarily want to cast Echo. In the case you are talented into both, then you will want to primarily focus on casting Emerald Blossom during fights and only use Echo in between pulls to apply Reversion, or when no damage is happening but you want to prepare for a large damage event that you can then combine an ability like Verdant Embrace with to top the party almost instantly.