DDP vs Apollo: scenarios with unreliable connections and limited speed

Which one uses less traffic and is generally better for online real-time applications in harsh connectivity environments? I.e. connection might be lost for minutes, hours, even days, while an application is running. All the diffs must be synced then as soon as the connection is back.

Another question is about local DB copies. With DDP we have (had?) GroundDB. Is there an option for Apollo? That’s for those cases when an application is quit by a user during connection’s down-time.

Thanks!