They both work. PCI requires a PCI bus (standard in desktop computers, nonexistant in laptops), requires you to crack the box and install. USB is plug and play, portable. Requires a USB port (not on older boxes), two versions of USB named (aptly enough) USB 1 and USB 2. If you have older box with USB 1 don't buy a soundcard that requires USB 2. PCI has a higher throughput IMO but I don't know if that necessarily translates into better music.