On a dead downwind course, the board speed is equal to that of the true wind minus that of the apparent wind. A planing hull is faster than a gliding hull.
From what I understood from Jim's chronicle :
The Apollo's 80 cm wide and last 30 cm short planing area means lower hydrodynamic drag for the lift provided. This means that the hull can start and sustain the planing which is faster in light 6-7 knots winds. The heavier the rider, the wider the tail must become to sustain the planing and speed.
The Serenity's 40 cm narrow and > 200 cm long "planing" area means higher hydrodynamic drag for the lift provided. This means that the hull cannot start nor sustain the planing, which is slow.
Try both with say an 11.0 sail, you'll be amazed at the board speed difference on a dead downwind course in 6/7 knots of true wind speed.
This is the same principle behind hydrofoil boats : the planing surface of an U-bent hydrofoil is very wide but very short to maximize the lift to drag ratio.