not sure about the new pc373d but i'm familiar with the pc363d which is a bass light headset.
usb headphones include their own dac (digital analog converter) and amp (amplifier) and bypass your onboard or soundcard completely. quality level of the one included is generally mediocre. if you have good quality audio products like a soundcard, this is detrimental. if you have low quality audio products this can be beneficial.
the quality of high end onboard audio is about the same as low end soundcards like the xonar dg. what is included in usb headsets can range from low quality onboard to high quality onboard. at best i would say its equal and at worst i would say the usb may be slightly worse. should be somewhat comparable but i'm more...