It really depends on what your in the mood for. The mocha is more of a true coffee flavor. The machiatta...well you are familiar with that.
I ordered the Mocha with my first order many many moons ago and fell in love with it. Then a week later i got the Machiatto and well, i fell more in love. I pretty much just stick to the machiatto now but when it runs out i will finish off the mocha and then order some more machiatto.