We have recently started selling the USB version of the ZBPLM and there have been quite a few people wondering about the differences between the USB and Serial Version.
Advantages of USB over Serial:
1. New Zigbee Module provides more range.
2. No need for serial to usb converters, the USB version will attach directly to the RPi and is compatible with RPi and ZBPServer.
Disadvantages of USB over Serial:
1. Costs $10 more. (Although since some serial to usb converters can cost about $25, the USB version can actually be a cheaper option).
So I would recommend buying the USB version for anybody using the Raspberry Pi. But if your application would benefit from a Serial Interface then it would be wiser to buy the Serial Version.