Mics need to be plugged into a microphone preamp, the better the quality of the preamp the better the sound will be since preamps have a significant effect on the recorded sound.
If your sound card has a breakout box with XLR inputs and good Mic pre's then running straight into it will have the advantage of keeping the 'signal chain' short, which minimizes possible signal loss and leaves less chance for noise to find its way in.
If the soundcard doesn't have high quality mic pre's then you will get a better sound if you go through an external mic pre or mixer.