We enjoy both ports, just know what you're planning in advance.
Costa Maya has lots of options at the port, we enjoy going to Maya Chan for the day. But as someone else said, beaches have been rough with sargassum. We have been twice, last time in December. They said the sargassum is getting worse for more of the year now. But the included transportation, private palapa area, great drinks, amazing lunch, etc all make it worth it. 🙂
Belize is a tender port, so know you may not make it if seas are rough. There isn't much near the port, so plan ahead what you want to do. We've gone cave tubing several times. The port has become more commercialized, believe it or not, in the past 10 years or so, but it isn't as shiny and clean as Costa Maya. Worst part about Belize is waiting for a tender back to the ship.