WiFi or Bluetooth (or USB) for iPad Tethering
Range


Wifi is the best
Bluetooth shorter
USB length of cable (there are 10' USB cables for iPad)


Security


USB is the best assuming you have decent physical security
Wifi is pretty good if you use WPA2
Bluetooth is the lowest probably


Speed (usually all better than your 3G speeds so a moot point for 3G)


USB is the fastest @ 480 Mbps for USB 2.0
802.11n tops out @ 150 Mbps
Bluetooth is by far the slowest @ ~3 Mbps


Power Usage


USB is the most power efficient, however the iPad will try to charge off your laptop
Wifi and Bluetooth are less power efficient than USB but I don't know how they compare  


Bottom Line: USB is the best if range isn't an issue, If distance is needed use Wifi with WPA2