The advantage to class D is that they are more efficient at lower volume when it comes to current draw versus your typical AB amplifier. Generally, a typical class AB amplifier will draw 50 to 75% of its maximum current draw at lower volume levels whereas a class D will draw less. Class D amplifiers also tend to run cooler, but this isn't always the case (*cough, choke* the new Alpine PDX).
As for which one sounds better on subwoofers... Until someone shows me a double blind test with statistical relevance, I am going to say that a quality class D amplifier compared to a quality class AB amplifier will not make a difference on subwoofers.
Why?
1. Because it is reproducing the area of the sound spectrum where the human ear is least sensitive to changes in frequency.
2. Because it is installed in a freaking car! What is the lowest noise floor to overcome? 60 dB in the most quiet of vehicles and 90+ dB in the noisiest of vehicles? Sure, I believe all these individuals who hear a difference between the different quality amplifier class topologies.//content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif