I believe Isa's case is "solved". When LE will press charges is another matter.
I believe Sierra's case will be solved quickly for the following reasons -
1. she was found quickly. This will make investigating the crime a lot easier - witness with fresh memories, evidence still in situ.
2. the dumping of her body so publicly and close to home implies to me that it is a local killer, and disorganised. He may be young as he doesn't appear to have had a place to take her to, nor means to take her. It is likely he is an RSO or has a police record of another sort that will bring him to LE's attention, or he will be suggested to police by someone who knows him.
Of course there is not enough released publicly so far to really make a judgement, but the above is my gut.
JMO!