here's my 2 cents.
because of the class d's topology, its designed to play a narrow band of frequencies, preferably lower ones. because the amp works with a reduced bandwidth, it can "pump" out more watts. ever notice most class d amps are single channel? more watts per sq inch. of amplifier chassis. than a class a/b.
class d are also more efficient than a/b and even more so than class a, which is the cleanest of the 3.
class a/b, is as it says, a class output topology, and class b powersupply design.
its one of the most popular design. reasonable quality and cost when compared to class a. class a/b are capable of playing the largest bandwidth of the three.
to summ up all this preamble:
class d: deeper, louder sub bass.
class a/b: tight, clean, clear lows, as low as you need..(depends on the brand)
if your're after sq. stay with a/b, unless its a really high high end class d.