The difference between these two is that, not surprisingly, metal sends the high frequencies across with a sharper, more brilliant edge - which you may or may not want, depending on the music. Component materials are just one of the many intersecting elements of speaker design, however. But actually, the speakers are the hardest part of a sound system to do right. That's largely because getting the signal through the drivers is just part of the job. What happens when the sound hits the air is the interesting part and leads to all kinds of interesting variations in design.
Speaker cabinets come in various sizes, shapes and textures.
Plastic is used more often than you might imagine, although wood still is a popular housing material. Now, depending on the way the cabinet is designed, this leakage may or may not be a major consideration. Any of those cabinet designs must be considered in terms of the room you're planning to put them in. Then again, you could make the room itself into the cabinet. But expect to start around twice that for an in-wall speaker with a pivoting tweeter.