I don't know anything about Isabella, but I saw Melanie several times a few years ago. Her gallery suggests that she's put on a bit of weight since then but she's probably none the worse for that. She was very willing, fun, with a lovely soft, smooth body. She gave excellent OWO and she could talk (entertainingly) for Wales. If you choose her I don't think you'll go far wrong.
On the other hand, Isabella does look extremely tasty.... On the other hand again, you might be better off going for the infinitely more experienced Melanie, with her lengthy track record...!