Iron stair parts -particularly iron spindles and handrails--last much longer than their wooden counterparts. Of course, if you're installing iron accoutrements for your staircase, make sure that you've worked out the measurements. It's much more difficult to retrain or cut iron balusters after it's been installed than it is to retool wood after the fact.
More and more home carpenters these days employ iron stair parts in their outdoor stairs. Although iron provides a more functional aesthetic, it also tends to weather better over time, provided that you keep up-to-date with your anti-rust coats. If your iron stair parts get dented or otherwise maligned, you should probably call in a professional contractor for the job--cutting iron correctly requires an advanced knowledge of soldering technology.

Building a Sturdy Stair Case with Iron Parts
The one danger with building an outdoor iron staircase is that iron conducts electricity. If your house is relatively isolated or high with respect to other nearby points, your iron staircase could act as a lightning rod. While the odds of a lightning strike on your staircase are slim, you can take precautions, such as capping your balustrades with non-conductive rubber.
Iron stair parts don't have to be aesthetically minimalist. Indeed, there are plenty of ornate wrought iron balusters, particularly for newel posts. Some companies will allow you to pre-order iron balustrades or newel posts to match your home decor. However, any time you customize your stair part requirements, you usually have to pay more.