Occultist is stronger. Possession also offers damage absorption, Blood of Dreeg also helps survivability, and Solael’s Witchfire significantly increases DPS.
Things like using two Deviltongue or binding Wendigo’s Mark to Fire Strike are definitely going to help with survivability by providing a large amount of leech.
I chose sorcerer (demo+arcanist) and happy with my choice. All supporting skills work pretty well with Demolitionist’s offense and you have enough damage to melt everything in vanilla (but haven’t actually tried nemesis yet, not enough infamy).
Sorcerer(+Arcanist) : All the leveling way was easier and happier. Easy to customize and obviously effective skill, but when come harder challenge during endgame such as slaying nemesis. It doesn’t really easy. You’ll struggle by the limit of class and gear.
So it might be harder in the end if you’re seeking for power, but anyway sorc is viable for many build and can switch to play caster easily.
Pyromancer(+Occultist) : A bit harder than sorc and need to know some defensive mechanic. You’ll be end up squishy if you don’t know occultist true potential and don’t know how to facetank properly. Because it doesn’t have stopper skills and mirror(3s immortal) like sorc.
During endgame challenge and making full build. It shine and easier to figure out because there’s very good synergy from both mastery in fire and chaos. Good gear support. So pyro can make the way further and stronger than sorc in the end.
PS. If your’re playing DW gunslinger
Sorc : you’ve easier access in lightning, some aether conversion items, more crit dmg and easy to use skills. Use it wisely
Pyro : you’ve better synergy. Better gear. Instant health, physical res. More resistance reduction->more dmg.