Because there are two US and two UK covers (and because I loved this book), I will be comparing FOUR covers instead of two!
Here they are:
The First US Cover The First UK Cover
The candle on the US cover is mysterious, and I really like the billowing smoke, but the UK cover foreshadows the book better with the lonely heart floating in the water. Though I don't understand why the "for" in the title is italicized on the UK.
The Second US Cover The Second UK Cover
Again, the UK cover is the winner. There's nothing wrong with the girl, but the daisy with specifically twelve petals is symbolic and I like both the title and author text. Notice how on the first US cover there's the sticker for the Printz Award, but not on the second.