Yes.. theoretically you WOULD get better quality from the sound card jack... but I personally use a USB headset.
1> Connections at front of tower.. easy to plug in and unplug (I only use headphones for AH.. for the most part anyway)
2> Powered up by USB jack. Some sound cards may not have a powered jack, which means the headphones would need an external power supply. Whether it be a cord or batteries... well either way it'd be more of a pain.
3> ON-BOARD sound. I do not use an external sound card, nor do I really wish to buy one for one game. I know some of you guys out there are bigger gamers than me, but personally I don't see much use in purchasing a SB Live or whatever, when my on board serves all of my non-AH purposes. Hooking a headset to on board sound may defeat the purpose, as many do not support 5.1 surround, and quality may not be that great.
Yes many people may argue that USB uses up "valuable" bandwidth and processor cycles, etc.. and that's probably true to a point... but I never noticed any difference when I first started using this headset.. and that was on an XP2200 system.. now I'm on a core-duo E6300 and it definitely doesn't seem to load down the processor any...
I will admit.. I get very slight interference sometimes at very low volume levels... but it's hardly noticeable, and for sure is not an issue during normal AH sounds.
I can tell you this.. whatever you get, make sure it's surround! Yes they make surround headphones.. I can vouch that they completely changed my AH experience.. more so than my LCD... more so than my joystick... even more so than my faster computer!
Good luck!