I have played both Demon and Sage seeker, and my best best best friend has too and personally they are both so good it is hard to choose just one >w<;
One thing I noticed though is that they have like two different fighting styles and that's what you want to decide on.
My best friend's playing style is more for Demon: he likes challenges so his bit lower defense to attack harder, he purifies himself when he does a little kiting, and he is quick on changing between his long sword to dual blades. He's a pretty good damn demon seeker.
I'm not a huge pker, I like pve more and the little more defense makes me feel just a Tad better when I face an arcane class lol.
Personally, do the two seekers on one account and try both cuz you WILL favor one more then another on your fighting style.