XBee Powerdown (using Pin 9) vs removing the power

Hi, XBee takes about 6uA when in low power mode. However, what is the difference of this mode verus that of turning off the device completely. From what I've read it seems that power down mode is the same as remove power mode. I could have a transistor and just disconnect the power rather than putting the XBee put into a low power state and achieve the same effect

Sleep mode on the other hand does what faster, but requires milli amps, not micro amps.

Thanks
Kiwi