What is the difference between a dual 4-ohm voice coil and a dual 2-ohm voice coil?
In actuality, neither is superior to the other. Simply put, the difference in the coil configuration helps determine the impedance (or resistance) an amplifier will see when the subwoofers are wired to the amplifier. You will want to consider the capabilities of your amplifier and the number of subwoofers you will be connecting per amplifier in choosing which coil configuration will work best for your set up.
Are single voice coil subs (SVC) or dual voice coil (DVC) subwoofers better?
Similar to the difference in coil configuration above, one is not better than the other. In today's market, it is more common to find dual voice coil subwoofers as they allow for greater options on the number of subwoofers to be wired together to achieve desired stable ohm loads on amplifiers.