Form Follows Function
Your best bet when deciding on the right materials for your outdoor furnishings is to evaluate how you plan on using your furniture and how much time you want to spend maintaining it.
Here are a couple of examples: A lightweight aluminum or plastic chair will be rust-resistant and easy to move around if you plan on dragging it into the front yard for the annual neighborhood block party or stowing it in the shed come October. It won't have the heft and stability of an iron or stainless steel piece, but it might be stackable (or collapsible) so you can hang it on a wall in an out of the way spot when you aren't using it.
On the other hand, a cedar loveseat will be very sturdy and feel more like the kind of furniture you're used to indoors. It will require added maintenance though, like a coat of sealer every couple of years, and moving it from place to place to catch some shade (or sun) could be a problem, too.
- Aluminum, plastic and PVC -- These construction materials are rustproof, lightweight, relatively inexpensive and require very little weather treating. You can also wash them easily with a little soap and water.
- Steel and wrought iron -- Both are heavy duty and sturdy but will rust if not weatherproofed or painted periodically.
- Rattan, wicker and natural grasses -- It's pretty amazing how well natural materials like wicker look and last outdoors, especially when they are treated with a resin finish. They may require additional weatherproofing every couple of years, though.
- Wood -- Natural wood looks very attractive in outdoor furnishings. It makes sturdy furniture that can be as comfortable as anything you use indoors. It does require regular treatment with a preservative and may also require UV protection. Choose weather-resistant woods like teak, redwood, cypress and cedar.