The choice of one configuration over the other is based almost entirely on the impedance of the load that you intend to drive with it.
National's amp chips generally are capable of driving 4 to 8 ohm loads (with proper power supply voltages and heatsinks, etc.). A parallel configuration will effectively double the impedance of the load that is 'seen' be each amp (since each amp delivers only half of the total current). That means that the same amp chips can now drive 2 to 4 ohm loads. A bridge configuration will double the output voltage, which results in double the current. This means that each amp chip will effectively 'see' a load of half the impedance of the actual load (a 2 ohm load draws twice the current of a 4 ohm load for a given voltage, simple ohms law). A bridged amp design is best suited for 8 to 16 ohm loads, since each amp chip likes to drive 4 to 8 ohm loads.
Once you decide on parallel or bridge, figure out what the effective impedance is for each amp chip. Design the power supply and heatsinking according to that, using the data in the datasheets. Power supply voltage is very important, and is very often overlooked by many DIY'ers.