For some classes passive skills
in Sage version give more dmg (for BM 15%)
in Demon version give more Crit (1-2%).
Demon Spark gives a bit more APS,
but Quicken Stone in your weapon can
give the same thing, it activates pretty often.
Sage spark gives more def and damage, but not speed.
Basically, Sage hits harder, Demon hits faster,
but that can sometimes be completely opposite.
So, the best is, as Antihero said, open list with
Sage/Demon skills and compare them.
Then decide what you like more.
On the other hand, here is pretty fast/easy to make
two toons of the same class, one Sage, one Demon,
level and gear them to, say, 140-145, and then see
which one you play easier and with more fun.
Then that one level to 150 and get endgame gear.
Other one delete, or keep as backup if you change