I think the important difference is that the Serenity is engineered for nonplaning conditions, and in addition, it's supposed to be a plug and play design. A Phantom, or a classic raceboard, is designed to use an adjustable daggerboard to present, moderate or hide depending on the amount of wind encountered. Also, they can handle planing conditions, and usually include upwind and planing footstraps positions, whereas the Serenity doesn't.
If nonplaning conditions are the goal, I'm thinking that the Serenity can dominant due to the concept design and much lighter weight.
I'm still waiting to receive my Serenity, but Starboard must be able to supply the standard Starboard protective bag. Something that they seem to be having a hard time doing. I hope they can get it together, because it's a real deal breaker for me. Marketing the board without the bag is a shallow approach, as it's such a long and complicated shape to transport, protect and store.