We enjoyed both ports. I didn't like having to tender in Belize, but we visited Altun Ha mayan ruins and did a river cruise as part of a Carnival excursion and had a great day. This was years ago so I'm not sure if they offer that excursion anymore. I believe you have to plan an excursion as there isn't much to do around the port. I think I would find a snorkeling excursion next time.
Last year we went to Costa Maya and took a taxi & spent the day at Blue Reef (all-inclusive). We loved our time there so much! The food was great, the drinks were good, and the hospitality was awesome. The only complaint I had was that there was a ton of sargassum on the beach...which I believe is an issue there quite frequently. Not the nicest beach, but we didn't let that spoil our day. We got cheap (but great) massages as well. The port area at Costa Maya is large and has several shops and a pool (I think). Some people just hung around in that area all day.
I might give a slight edge to Costa Maya if it were me.