WiFi or Bluetooth (or USB) for iPad Tethering

Range

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

Security

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

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

1. USB is the fastest @ 480 Mbps for USB 2.0
2. 802.11n tops out @ 150 Mbps
3. Bluetooth is by far the slowest @ ~3 Mbps

Power Usage

1. USB is the most power efficient, however the iPad will try to charge off your laptop
2. 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

I just did some speed benchmarks using all three methods. Using WiFi and USB gave around 8.75 Mbps, which were extremely close to the result of doing the same speed test on the phone itself beforehand, but Bluetooth was much slower, at around 0.5 Mbps. The difference was very noticeable when browsing.