Comparison of the efficiency of DC motor current limiting / control methods?
So how do the above methods compare in terms of efficiency and
  simplicity of design?


The L293B (or D) is really a limiting factor on this design. When used in a H bridge configuration from a 3.6 V supply, you'll be lucky to get more than 1.5 volts across the motor. This is because of the turn on saturation voltage in the driver transistors. Check out this data sheet and look for \$V_{CEsatH}\$ and \$V_{CEsatL}\$ on page 4 - they tell you that at 1 amp the top transistor saturation voltage is typically 1.4 volts an the bottom transistor saturation voltage is typically 1.2 volts.

Upshot of this is that you can probably use a much smaller motor for the steering if you used a better driver. See this for a more complete explanation.

Using a better driver will also improve traction, power and efficiency on the rear drive motor too.

Another option for the steering is to use a servomotor - this can be positioned to just avoid mechanical end-stopping and therefore reduce current consumption.