There use to be problems with the 80211b Wireless USB adaptors. The 801.11g are stable and work well.
PCI might be a little faster than USB with good external Antenna. However, PCI takes an IRQ, which might pose a resource problem in some systems.
As a rule of thumb if the Wireless client is in a close proximity to the source. I usually use USB, if distance might en an Issue I use PCI that give me the flexibility of good omni or directional Antenna.
:sun: