Two totally different experiences. You can see the zoo easily in 90 minutes or less. And, it's not the stereotypical US type presentation either. Personally, I'd not make that a day trip stop, but save it for a stop off on a multi day mainland visit.
Take Law's & Simon's advice - Lamanai, definitely. Great photo ops for the hubby. Wonderful scenery to/at/from the river drop off. Ruins set in the jungle, lush tropical foliage,howler monkeys in the trees above your head ( watch where you stand, unless you have a "rain hat"
..it doesn't get much better.