Cored or NOT cored?????
To begin with, whether a hull is cored or not tells you far less about a boat''s strength and durability than how that boat is framed. You are also ignoring the choices of building materials and methods. For example there are huge differences in strength and durability between a thatis vaccuum bagged and laid up with vinylester resin, no non-directional fabrics, a moderate to high density foam core and a boat laid up in polyester resin, high non-directional cloth ratio with or without a glued core. But to answer your question;
I strongly prefer a cored deck but especially on smaller cruising boats. On boats under 40 feet the design loads are determined by the weight of people walking and jumping around on the deck and the framing spacing is very similar between a large boat and a small boat. As a result the deck lay-up schedule does not vary much between a 20 footer and a 40 footer. In either case, to achieve the necessary stiffness, a glass deck without coring would weigh over double what a cored deck would weigh. This is not the end of the world on a 40 or larger footer where it would represent perhaps a 5% increase in the weight of the boat but on a small cruiser this can add 15% or more percent to the weight and since small boats are much more weight sensitive to begin with and have a harder time carrying sufficient stores etc.,on a small boat a deck without coring would be a really bad idea.
If durability is a concern, the ideal solution is a cored deck constructed using vinylester resin and no non-directional or cut fiber reinforcing with a moderate to high density closed cell foam core. While a moderate to high density closed cell foam adds cost and is roughly 10%-15% heavier than Balsa (adding something in the neighborhood of 1% to the weight of the boat), it is inert and can withstand the presence of water without delaminating. In long term and impact testing this has generally proven to be the most durable and impact resistant construction form of fiberglass construction.
As to hulls, I strongly prefer a similar construction to that described above, (i.e. vinylester resin and no non-directional or cut fiber reinforcing with a moderate to high density closed cell foam core) that comes with some caviats. Areas of the bilge and along the centerline, areas near the keel and support, areas where thru-hulls and rigging hardware pierce the hull, the core should be left out and these areas have a extra layers of laminate built up. This should be done by tapering the core at the edge of the un-cored area so that the inner skin can be directed bonded to the shell skin, and then the area is further built up with additional tapered laminates.
If I were designing a boat for durability and strength I would add kevlar to the outer skin. In testing, vinylester resin and no non-directional or cut fiber reinforcing with a moderate to high density closed cell foam core proved to have a much greater impact resistance than any of the materials tested (including an uncored hull). Adding kevlar to the outer skin greatly added to the localized impact resistance.
Another issue is how the hullis laid up. Vaccuming the hull during ly-up produces a denser, stronger, more durable, and lighter layup. Vaccuuming the core and adhering the core with a higher strength, lower permiability resin like vinylester greatky increases the likelihood of a trouble free hull.
More important than whether the hull is cored or not is how it is framed. Ideally any FRP boat has a series of closely spaced longitudinal and athwartship frames supporting the skin. This has a far greater impact on the durability and strength of the boat than whether it is cored or not.