Let your agent know what you want when you want it and let them do the work for you! Takes away the hassle and the worry while letting them take care of everything.
As for the Miracle, we've been on it and its sister ship the Pride. Rather small ships and not alot of amenities, but we preferred the Pride.
Personally, if I was celebrating a nice anniversary like this, I would prefer to upgrade and do Princess.
But then again, all of these are relative long cruises and spend alot of time at sea. If you don't mind that, it's okay. But if you really want to spend time seeing the island and enjoying Hawaii, you might want to consider the Pride of America. We did that for 7 nights around the islands and it was a great way to see the ports and spend more time in each port than the roundtrip cruises out of California. It also gives you time to really visit Oahu. Just a thought.