What about who’s more at fault for the teenagers’ predicament? I would say Friar Laurence. He knew that Romeo was fickle; he chided Romeo for his rapidly changing affections, yet he helped the two anyway. The nurse simply thought it was romantic.
I think it depends on if you were Romeo or if you were Juliet, as I believe the original question asks. Romeo would, of course, be more apt to trust Friar Laurence - he didn't know the Nurse from Eve and so why would he trust her? Same goes for Juliet, although Juliet at least had a passing acquaintance with the Friar, and since he was supposedly a man of God, she probably would have felt as if she could trust him to some degree. But Juliet's natural tendencies would have been to trust the Nurse, the woman who raised her and who initially helped her in her schemes to get together with Romeo.
I think the Friar (although he made a stupid decision with his plan) at least had good intentions. The Nurse seemed trustworthy at the beginning and was more of a mother to Juliet than Lady Capulet. After Romeo is banished, the nurse tells Juliet to forget Romeo and marry Paris even though she knows Juliet is in love with Romeo. At this point, Juliet and we as readers know we can no longer trust the nurse.
In some ways, the Friar is a kind of "doctor" of the times... what with his potions and such.
It's very difficult to say because both of them are somewhat untrustworthy, but at the same time both of them present the only adult figures who offer to help Romeo or Juliet.
Know what we know now, it's obvious the Friar's hare-brained sleeping potion plan is the worst overall advice that someone could have provided.
However, given what Romeo and Juliet know during the course of the play before the Friar's scheme, the Friar does seem to be more trustworthy, afterall he is a man of the Church and seems to be one of the only people who support their love.