Hi I take it you are talking about the Iberostar hotels here......both have got excellent reputations...you might want to consider also the Iberostar Creta Mare and Creta Panorama that are right next door to the Creta Marine (and have a slightly better reports from what I've seen).
I'm not sure about the Mirabello Beach, but the others are all in an out of town location on a headland - wonderful views and well connected for transport to nearby towns.
If you read the reports about the Creta Mare & Panorama, that should give you a fair idea about the Creta Marine, right next door ( they are all run by the same company), which does not seem to have as many reviews. They also share some of the same facilities.
We stayed at the Creta Mare / Panorama (they were being run as one hotel) and it was excellent.
Depends what you are looking for though

..........if you are looking for a lively hotel in the middle of things, with a sandy beach, these would not be for you...but if you are looking for good quality accommodation with wonderful surroundings and great food/drink...then you've found it.
PS. Chix1 has stayed at the Mirabello...perhaps you could PM her for more details if she does not see this post.