I know I'll probably get flamed for this and it may seem funny, but maybe I can shed some scientific light on this subject with an unrelated application. I'm a dental professional with biomaterial experience, specifically dealing with the material science of dental restorations. Anyway, we pretty much use the most technologically advanced materials in use today - anything from polymers, resin composites, all kinds of metals and alloys, ceramics, titanium, ect.
The materials we use have to be able to withstand one of the most hostile environments on the planet. The oral cavity is pretty much drenched under water 24/7 with all kinds of enzymes, bacteria, and most of all - teeth endure extremely high forces on a daily basis. A single tooth cusp undergoes repeated stresses of up to 10-40 million pounds per square inch all day long. So to make a long story short, dental materials have to undergo stress analysis and must meet very high standards of mechanical properties. Decayed teeth are restored with these materials that have to withstand the constant stress and abuse of the oral cavity, and they have to last decades. The restorative materials themselves actually outlive the biological material they were meant to replace (due to recurrent infection and decay), and failures are most often not due to the ultimate strength of the material, rather they are due to progressive fracture under repeated loading (thousands and millions of times.)
Two of the most widely used materials I use on my patients are composite polymers reinforced with all types of filler particles. The other material is a metal, or metal alloy. Composite polymers have high fracture toughness, tensile strength, compressive strength, and yield strengths....they are pretty amazing materials. They resist corrosion, but DO wear over time due to the high hardness of opposing tooth enamel. All in all, they exhibit some properties that are actually better (and some worse) than metal at least for dental biocompatability.
I will say this...the metals we use in the dental field are tried and true. If I had to put a restoration in my mouth, it would be a high noble gold restoration. This is probably a stretch to compare to guns, but I figured I would try and make the point that both are excellent materials and will outlast the life of its user in most cases. Depending on the type of metal used and finish, corrosion IS an issue - and left uncared for metals will usually corrode faster than polymers. However, if the finish on the metal is maintained, I would take the metal for its optimal properties over polymer.
Another point to make...no matter how ideal the conditions or material used, no dental restoration is said to be permanent. Given enough time, it will eventually fail (it is simply a mechanical device), and all machines will eventually have failures given enough time and use.