There are five Voyager class ships.
Voyager, Adventurer and Explorer are designed with the balconies within the structure of the ship. Thus the reason for all the steel or other equally strong hard to see through material.
Navigator and Mariner are designed a little differently. All of the balconies are outside the structure of the ship. Thus no need for strong to see through material. Thus the balconies on these two ships are more open and have plexiglass on the railing.
There are some other differences between the ships. For example the Navigator and Mariner do not have a water slide. Instead each has a much bigger area for the teenagers.