Personally I trust Peitha more than I trust Isgarren. Peitha has saved our lives multiple times, been seemingly upfront about everything, and hasn’t really given us a reason to not trust her.

Isgarren on the other hand has an underlying god complex, and I feel will betray us without hesitation for the ‘greater good’ of Tyria. Or you know Anet could be doing the whole swicheroo where Isgarren says he is not possessed but he really is possessed and is just playing along to lead us on to kill us at the perfect moment.