When seekers were first introduced, everyone immediately sidelined being demon because of all the benefits Sage offered over it. As time went on peoples minds have changed.
Demon Vortex is seriously OP for PVE, but with its long start up time(the animation and the delay before you actually do damage), I'm not sure how vortex period can be any factor in PVP.
That said, Edged Blur would be ideal to depend on. It's great damage, about the same range as vortex, and best of all, you can move while it is in use and perform other skills.
Sage or Demon, doesn't really matter. With all the tools the class offers, you'll get a sense of safety with the class.
And well, I'll toss this in there too, with cloud eruption, you are capable of having vortex and edged blur active at the same time.