Played both demon and sage mystic for quite a reasonable time now and my personal preference is to use sage over demon.
Sage:
Has higher base p,def with the verdant buff as well as having a higher base damage. The 25% chance to purify on the basic heal (which is used very often) is extremely nice too.
Demon:
Demon grants more chi by summoning pets as well as having higher channel off skills such as rapid growth. Verdant shell provides a 15% tidal which is nothing and will not save you in PvP compared to the extra p.def.
My recommendation would be to go demon until you have a high enough base channel, once your rapid growth puts you over 100% channel with attack weapon I would then switch to sage. With 6% channel on every rank 8 piece the extra channel from a demon mystic is useless and actually slows you down.