I'm not a fan of that spelling, but the first one. Joanna looks better, IMO.
Shiloh reminds me of a dog.
This is why it reminds you of a dog.
And given that it's a classic, award winning book, there's a good chance your daughter will read it in school. And it will make her a very easy target amoung her peers.
Don't be cruel, go with Johanna.