I've tested Demonbolt and Soul Conduit yesterday in my mythic raid.
It feels that Demonbolt is the stronger option, but i see many other guys picking soul conduit and performing very well too.
What do you prefere, on which encounter you choose demonbolt or soul conduit?
Soul Conduit is currently the strongest option on single target when using the T18 4-piece bonus and Fragment of the Dark Star trinket.
Once you get to 110 it's not really an option if you're being serious about your dps. At 110 you are basically choosing between either Summon Darkglare or Demonbolt. The deciding factor is typically based on whether the fight is AoE/Cleave vs ST.
Summon Darkglare (which you should pair with Hand of Doom) fits into fights with sustained AoE/cleave (e.g. Xhul'horac). Straight up ST fights and demonbolt wins out but not by much.
Frankly I'd get used to playing with demonbolt since its what you'll end up using more often.