At the moment, the Choice of USB is minimal (Only two offers) in the next, few weeks there will be explosion in network USB modules. Most of the big player will come out with new staff.
I personally prefer the USB:
1. It does not take any IRQ, and it is very easy to install.
But more important
2. If the laptop is not very close to the Network Transmitter, and you need to play a little with the location of the antenna the flexibility of the PCMIA is minimal.
The USB consist of a small unit that connect with USB cable to the Laptop USB port, so the USB receiving module can be put in better location (within the length of the USB Cable). While the laptop is on your lap, or desk, the receiver can be at top of something, and have a stronger reception, the speed of Wireless Network node is determined by the strength of the reception.