both are amazing, some main differences:
FX700 (off memory):
-more 'airy' in its sound, they have huge vents on them
-the mids take a slight backseat to the bass and treble as those are the main focus of them, altough mids still has incredible micro-detailing
-not sure what the technical term is, but they have this awesome 'crunch' in their sound
-bass is great, textured, deep- up untill the fx200's I thought it was the best on the market in that price
-great imaging, placement of sounds is awesome
FXZ-200 (no burn in yet, probably about 50-60 hours on them of daily use):
-BASS, they have subwoofers in them, more textured and goes deeper than the fx700's (which is a reaaaally hard thing to do). Bass a bit loose at first, which some people might prefer, but as burn in occurs it looks like it'll tighten up and get more controlled. Best bass i've heard in any iem, and holds its own against many full sized phones
-each frequency is in your face, nothing takes a backseat. its a triple dynamic driver IEM and it shows. those subs only come out when needed, but in no way bleed into anything else.
-incredibly detailed mids, lush, crisp
-vocals in your head, less about witnessing a stage and more about being on stage
...just some thoughts