First of all, mBTC stand for mili bitcoin and uBTC stands for micro bitcoin. Now crack down the mathematics of bitcoin units.
           
            mBTC = mili bitcoin = 1 thousandth of a bitcoin = 0.001BTC
            uBTC = micro bitcoin = 1 millionth of a bitcoin = 0.000001BTC

We should switch to uBTC and be done with it (forever). Switch our clients, our exchanges, our store plugins, etc, to use uBTC by default