Which uses more battery when tethering - Wifi or BT on iPhone 4S? I'm not really bothered which is quicker - I'm guessing this is wifi anyway. I just want to know which would allow me to get the most life out of my iPhone 4S when tethering to it with may MBP. Thanks!
Speed is primarily limited by your 3G network connection.

Battery wise, Bluetooth should use less power (especially in the 4S) but the difference may be quite small anyway.

If you tether it with a MBP you may be better of with using the cable between iPhone and computer.