They're both very good places and excellent honeymoon spots. I'd just make these little comments:
* You might find Portofino's service a little more personal. I know that has been the comments I've gotten from guests and also that's been my experience when I've visited, as the owner-managers at Portofino are young and enthusiastic and very personally involved in running the hotel. Not that Phillip and Nadia are not personally involved at Mata Chica, but when I visit there on inspection visits for Fodor's or another guidebook I usually deal with a manager. Course, it could just be timing.
* For a honeymoon, privacy is important. Mata Chica's cabanas are single units and do offer privacy, but some of Portofino's units are duplexes. So it might depend on what is available at Portofino on the dates you need.
* Mata Chica is definitely more of a high-design kind of place. Down to the menu designs and the bathroom sinks. Very sophisticated. Portofino has more of a casual atmosphere. On the other hand, I was impressed by some of the little details at Portofino, like the way the housekeeper (or someone) had arranged flowers and shells for arriving honeymooners. (The Lodge at Chaa Creek does the same thing in its honeymoon suites.)
* Mata Chica's restaurant is more expensive.
* Portofino is newer. Mata Chica's units are in good shape, though when I was by there this summer the paint on the exterior of the cabanas had faded a lot from the sun compared to the last time I was there.
* Surprisingly, neither has an in-ground swimming pool, which is becoming a must for resorts all over Belize.