A few days ago she'd have said that obviously negative channels are more useful. Sure, it's nice to be able to heal people sometimes, but you can just prepare healing spells if you think you'll need them, you're not usually going to need to heal a whole group of people at once.
And the thing about negative channels is, you can't pick and choose who you hurt. If, say, a pair of guards are trying to arrest you, and there are bystanders in your channel area, they're a lot less useful than if you can plan in advance and make sure no one innocent rushes into the room until after you channel. She's actually hardly ever actually used her channels, not since the day she got picked. She spent a couple of them on the road, when wildlife got too close for comfort, but she could've done that just as well with spells.
And it's not like positive channels aren't useful. Raimon channels positive. Healing people who need it is nice to do, if you can. It'd help a lot with self-defense lessons, actually, if she could do more than just heal up two people a day. Once she makes Good she'll be stuck with positive channels, and that's not awful, there's lots of things she'll be able to do with them.
But it also feels like she'd be losing something, if she channeled positive, if she couldn't turn her first-circle spells into harm at a moment's notice. (Do those always go together? She thinks so, but she's not sure one way or the other.) It matters that she can hurt people if they deserve it, and trying to stab at them with her dagger just isn't the same.
She doesn't actually remember picking, exactly, she just spent her hour of prayers thinking about how much she wanted to hurt every last person in her lord's family, and when she reached inside herself to feel her new powers, there it was.
And negative channels were definitely a lot more useful on the day Calistria picked her. ...Admittedly, there's no good way to fully explain that without admitting to a capital crime, but she doesn't have to tell the whole story.
"They were more useful for me the day I got picked. I don't know if they're more useful in general, I'd actually never seen a positive channel until I came here for the convention. Probably it depends on the circumstances? But it's not like there's a way to change it, once you've decided, except by becoming Good, which, I mean — obviously I'm trying to do the right thing—"