in 2008 there was no advantage to the fx4 over a 4x4.
fx4's have 3.73's for 04+
lariat 4x4 with have same specs minus rancho shocks as a fx4. possible different gears depending on the truck.
2009+ the fx4's have a locking rear end which the other 4x4's don't up until 2011.
ofcourse there might be some minor interior differences, lariat will have wood grain while an fx4 has carbon fiber on the dash. gauge cluster might be slightly difference, etc.
sure theres a few other minor things i'm missing but if the locking rear end isn't a big deal (it shouldn't) i'd just find a truck that fits your specs and add a selectable locker yourself if it's something you really want.
if you choose a fx4 over an xlt, the fx4 (08) will have an option of a center shift console for 09+ all fx4's (i'm pretty sure) had the center shift console instead of on the column.