They're both deluded, but Isran is probably more dangerous in his fanaticism. Without Dovahkiin's help, Isran will continue his crusade - but what can Delphine do? Go kill a dragon? Well good luck with that.
However, if you're simply asking who's mory bossy, Delphine all the way. Isran actually is the boss, so he has every right to be annoying.
Who is Isran? If he belongs to Dragonborn, this topic should be in the Dragonborn section.
He's the leader of the Dawnguard.