IIRC, bluetooth maxes out at 1MB/S so you will definitely get better speed from USB (not to mention that Bluetooth will probably flatten your battery in pretty short order!)
It seems that Bluetooth 2.0 + EDR has a transfer rate of 3.0Mbps. I can't find anything that indicates Bluetooth 2.1 having a higher transfer rate. So if you did indeed get a connection greater than 3Mbps on your iPhone then the bluetooth tethering could be a potential bottleneck.