QWIK..no to be an *** or anything but this question has been asked at least once a month ( it seems ) for the last 2 years. A quick search should give you plenty of info on it.
In short..Lariat has more leather in the seats then the FX4, Lariat has the message center that shows the fuel consumption and crap like that. FX4 has stiffer shocks ( Rancho made ) and thats about it. However, you can get the FX4 with the convenience package and get all the little extras that come in the Lariat.
FX4 looks cooler, looks meaner and is just better all around...Get an FX4