Lamanai is a nice trip, but there's no comparison. As long as you're going all the way to Lamanai, may as well go the rest of the way to Tikal. If you're going to do one Mayan ruin, make it Tikal. If you haven't booked your trip yet, you may want to schedule a night in Tikal before or after Ambergris so you don't have to backtrack.