Metal and wood have different advantages as materials for exterior doors. Here are a few to consider.
Nothing quite says tradition more than the solid character of a wooden door. They tend to be solid by design, heavy, artistic, and reliable for many years. However, there are some weaknesses inherent in wooden doors. Hinge attachments can become worn out from repeated action and stress. Exterior conditions can impact the longevity of the painted, varnished, or urethane finish. The most common problem facing wooden doors is their nature to expand when exposed to moisture, causing doors to bind and become difficult to open or close.
This being said, let's not forget the pure beauty of a wooden door: the solid, secure feeling it offers and its flexibility with designer hardware installation and attachment.
Metal doors may save you cost and will offer you a consistency wooden doors may not. They are designed and manufactured to be a perfect fit. Hinge attachment won't wear loose from repeated action. Doors are solid enough to dismay any intruder from breaking in, and locks are not easily removed or broken out.
The downside with metal doors seems to be their design aesthetic and ability to be creatively finished. Metal doors will require priming, and painting in a solid finish—no staining or wood grain to encourage here. Only solid colors will do, and the application with metal doors should always be sprayed on. Brush, sponge, or roller painting on metal doors inevitably will show lap marks and look unprofessional. Also, beware that most metal doors do not have crisp, hard-edge design contours found in wooded raised panel design doors. Also, metal doors are pre stamp-cut for lock installation. There is no flexibility here with regards to designer hardware installation or placement.
So there are considerations to be made. Beauty, traditional design, and the flexibility with designer hardware installation found with wooden doors. Or the durability, consistency in dimension, and cost savings found with metal doors. Keep in mind that both door types will require finish maintenance from time to time.