Which uses more battery when tethering - Wifi or BT on iPhone 4S?
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.
Although Bluetooth might use less instantaneous power, it will be slower and thus leave the radios turned on longer. Having the cellular radio turned on for data tethering will burn the most power by far, especially if you are far from the nearest cell tower.