At the risk of subjecting myself to the ire of the 'yard, I think the starter is Castle and I'm not sure it's close. I doubt Hurley is losing sleep over it.
Solo is playing SOLID for a freshman coming to a top-5 team and projects as a nice piece going forward. He's going to learn to pass better, drive harder, shoot more consistently, and end up an All-Big East-level player if he sticks around that long.
However, at the end of the day Castle is more versatile on defense while remaining fundamentally sound, and is a leader on offense rather than floating for most of the game. There's a reason why he's projected as a top-10 pick. He's almost a complete player at 6'6 if not for his lackluster shooting.
Both of them need to start shooting the rock more consistently for us to reach our ceiling IMO.