@Daphine I dunno. The detective, however odd and perceptive, and no matter what he promises, has certain drawbacks. First, he's part of the general matrix of authorities that have backed her parents, been useless at best and in one case a psycho killer. She has no reason to trust someone from the world of officialdom. His relationship with those authorities is a bit odd, but still. Second, he's too damn clever and smug. He's displayed an ability to lie with a straight face, to say various things just to do things like provoke her to get information. It's hard to tell what he's really after, and whatever it is his demeanour suggests it has more to do with satisfying his ego than doing her any good. Third, the mere fact that he's making really big promises is a strike against him--everyone she's ever met who has talked big about how good they will be to her has been lying with evil intentions.
(In actual fact I think his intentions are good, but he's so in love with seeming clever that it undermines his attempts to
seem like his intentions are good)
So yeah. Just because he says he'll do something nice doesn't mean he will, or even can, actually do it. Mask-kun on the other hand has a solid track record with her, does not represent the kind of person who has victimized her in the past, and has tried hard to avoid over-promising. So even aside from feelings, there's a case to be made that going with Mask-kun is a better bet than going with the detective (and by extension the cops, child care authorities etc).