wireless USB adapter. definitely. because it's a lot more portable, should you ever decide to change PCs, or bring it with you to work, or ever get a laptop, etc.
is it just as fast as the PCI card? (assuming that the USB card and PCI card are both 802.11g)
same speed. the network is going to move at a MAX of 11 MB/sec of 54 MB/sec, the way the card is installed has no bearing on this whatsoever.
Wait this is more accurate:
"With up to 130 MB/s transfer rate in both directions the same basic performance is provided on a notebook as a PCI card would offer on a desktop computer."
Taken from http://www.rme-audio.com/english/hdsp/cardpci.htm
look at ian's response. it doesn't matter how fast a PCI card can transfer data, the networks moving at a max of 11 MB/sec