Why use a crate:
When used properly and not abused, crates are fastest and most humane method of housebreaking dogs. Dogs are den animals that will instinctively seek out a small, cozy place to rest for shelter and security. Crates act on a dog's instinct to keep its den clean and unsoiled. Keeping your dog in a crate teaches him to "hold it" until he can go in an appropriate place, which helps your puppy develop his bladder control. For older dogs, crates act as a "bedroom" where they can relax and escape hustle and bustle of kids, other pets, and everyday family life. Also, it's never too late to crate train! A crate can help break bad habits that improperly trained dogs may have developed over years.
Plastic or metal?
Plastic kennels are good for travel, and are generally only method of containment permitted by airlines. Metal wire dog crates are ideal for home use, however. They are generally roomier and more open. They're also sturdier, so they will stand up to your pet's movement/ activity and be able to last as your dog's permanent bedroom.
A crate should only be big enough for your dog to stand up, turn around, and lay down. If crate is too big, your puppy will have enough room to soil one side and sleep in other, which defeats purpose of crate. Buy a crate large enough to accommodate your dog's full-grown size and get a divider panel. This way, you can keep one crate and have it "grow" along with your dog.