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Metal
From Bronze to Tin
Metal does tend to heat up, although painting it white will help to deflect the heat from some of your poor plants before they bake. The biggest problem with most metal containers is that they lack drainage holes. So your plants could theoretically drown in hot water. If you can't make a drainage hole, you will want to put lots of gravel or styrofoam in the bottom of the pot to help create a place for the water to drain to.
Many metals are also prone to rust so they may seem permanent but turn into leaky sieves before your eyes - unless you take care to prime and paint them. Of course copper and bronze don't have this problem - instead they develop a lovely patina.
Since metal can also be heavy, if you have a container without drainage the best thing you can do is use it as a sort of cache-pot to hide a homelier container inside. That way if you need to move them you can do pot and metal container separately and save your back.