A steel door can be best choice for security and also to the durability. But not always this depends on many material that use to make the make the steel door. Any of scratches, dents or else are easily to take care. Steel doors with other hardware are cost close to wood doors.
Normally, steel doors can add up with wood for the frame (ex - pine wood frame). Sometime wood frame can kill your security of the door. Steel door mostly will fill any cavities with foam. The skin can emboss with beautiful pattern or just make it smooth. Steel doors mostly are coated baked-on polyester finish (more info [1],[2],[3]). This required repainting. A better version will coat with vinyl this helps weather resistance, stainable wood fiber coating and some high end can be a laminated-wood veneer.
Steel door are usually a part of prehung system. So that the hinge area on the door must match the hinge area on the existing door frame.
Finally, you must check the warranty because some manufacturer will avoid this when you install with some others type of doors.
Referancne
1. http://www.thisoldhouse.com/toh/article/0,,213754-3,00.html
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1. http://www.trellicor.co.uk/pws/steeldoorsDescription.html
2. http://www.trendir.com/archives/001227.html
