They're both written really beautifully--simple, achingly painful and occasionally poetic descriptions. They both reveal the past tragedy slowly, like peeling off layers of an onion.
I'd have to say that I liked Light on Snow more because she got the voice of a little girl just hitting puberty just right. She has moments of hysteria, and outlandish but earnest daydreams, and that impatient desire to just be like everyone else.
Saints at the River seemed kind of contrived to me. One of the deaths seemed meaningless and pointless, and the romantic relationship the protagonist has seems to make absolutely no sense. Or it does, it's a very juvenile sense. It's the literary equivalent of that boy who impulsively kisses you on the cheek in kindergarten then runs off.
But I did appreciate the setting a lot. I don't think I've ever read anything set in the Appalachians. I loved that. It's like a foreign country to me. That's where the tourists need to go. Why go to the big cities? Big cities are the same all over the world. Places like that are uniquely American.