You may be better off with &[String] as a read-only view of Vec<String>. To get &[&str] I think you need to create a new collection to hold the &str values. (String and &str have different memory representations.) But the choice depends on what you want to do - maybe providing &str values adds a convenience that is worth creating a second collection.

For the Option case I would go with Option<&[String]>. My understanding is that Option<&T> is the same size as &T for any T so an owned Option wrapper is zero-cost. If the reference pointer is null then Rust interprets that as None. Besides you usually want ownership of an Option so you can map it or whatever else you want to do.