They both sound good, but let me emphasize shortcoming:
My problem with FX500, it bumps upper vocal, sometimes feel "funny / not quite right"; even more strange, it lost details including mid and low; sure, it has deeper bass (kind of lonely bass) - imaging this, the sounds dip/fall from mid directly into low bass, like a gap. The high is slightly harsh, which I can not listen more than an hour.
C710 sound, overall bright; nice bass (layers of, which bass should sound like), higher bass also carry into lower mid, plenty details for mid; I won't call the high harsh, but it's bright. Maybe because the transition from mid to high is more "connected", it does not bother me.