Unobstructed viewing, as mesh is too obstructive/cagey and glass, more often than not, gives a terrible glare. Plus, if the apes have access to the moat, (so more of a ha-ha or a ditch than a true moat lined with hotwire), it's more unique terrain for them to traverse. Lincoln Park's gorilla exhibit is a good example, along with Woodland Park or San Diego's respective gorilla habitats. I understand the usage of mesh for smaller primates, and definitely so for orangutans, but I see it as less necessary for gorillas.