Doubles can be faster than singles. Doubles may be able to carry more gear, but keep in mind that they need to carry more than twice as much gear for this to be true. A double will require the use of a rudder to steer. Some doubles are more stable than a single but will be more difficult to rescue and pump dry. There are lots of different hatch designs out there.
Considerations when looking at hatches are watertightness, resistance to breakage, and size.
If you want to bring the kitchen sink, you'd better not just have a 9 inch round hatch. Consider also that heavy seas and surf can break or blow off hatch covers, so consider how they are attached to avoid losing them, and don't depend on them for floatation of the boat. If the compartments aren't full of gear, use float bags. The cockpit can also be used for cargo, but keep in mind that it may not stay dry, it may impede your exit if that becomes necessary, and it may fall out if you do exit. Keep in mind also that a leaky hatch or bulkhead may compromise the watertight cargo compartments, and pack accordingly.