I have been using the starboard 9.5 sail with my Phantom 320. With carbon mast and boom it is very light and easy to use up to 15 knots. Depending on where you sail, the wind will probably be below fifteen knots most of the time anyway.
Don't be afraid of the bigger sail if you want to sail in light winds. The phantom 320 is a very stable board which also makes it easier to handle the large sails.
Just keep a smaller sail handy if the wind gets too strong. I think that your choice of smaller sail doesn't really matter as long as you can handle it. It is mostly the bigger sail you need to choose carefully.
Having sailed a few hybrid boards, the Phantom is much closer to a longboard design than a formula in the way it sails. I think it bearly qualifies as a Hybrid. As far as how that influences the sail choice, I would say that the more important factor is whether you want to sail it in subplaning conditions or not. The longboard sails will definitly give you more power when not planing and maybe get you planing a little earlier. The compromise is usually that they may be harder to handle when the winds gets strong, hence the smaller spare sail.
Last edited by tonymatta; 18th December 2009 at 10:24 AM.
Reason: more info.