Neither of these have nd.
Pyroblast hits A LOT more than flamestrike, so it does vastly more damage per second against stationary enemies.
Flamestrike does have some other advantages however. 1) It can be precast so you can summon stuff on top of it. 2) It's better against those bosses which teleport away when struck like astro, QT, brach etc. 3)It can be targeted across impassable tiles like moats