Metal
For the most rugged durability in your name brand designer sunglasses, metal frames will serve you well. Metal frames do have the ability to hold stronger hinges and thus hold your metal sunglass frames more firmly to your head. Metal sunglass frames will usually have a soft rubber nosepiece and the ends of the temples will usually be rubber or plastic coated for comfort.
For extreme sports competitors, the Bolle gun metal sunglasses can lend a look of confidence when you take the field. A division of Bushnell, Bolle is a top producer of quality metal framed sunglasses for sportsmen the world over. They have designs to fit many sports environments and are reasonably priced compared to some designer sunglasses.
Oakley is probably the most renowned brand of high performance sunglasses in the world and their Oakley "X" metal sunglasses are designed to keep them there. Since their inception in 1977, Oakley has repeatedly pushed the limits to design the best styles and most durable metal sunglasses available.
Ray-Ban of course continues to challenge this reputation with their high performance line of sports oriented metal sunglass frames. Their aviator style metal frame sunglasses are among some of Hollywood's favorite shades.
If you plan on extreme activity you may want to put out a little more for a set of Gargoyles metal frame sunglasses. Gargoyles have the contract to supply the military with the sunglasses for our soldiers. So the frames will be rugged and the high-density lenses are made stronger than bulletproof glass. In fact Gargoyles have been tested and the lenses have withstood direct hits from .22 caliber bullets.
If you don't want to be out the money for a quality metal frame pair of designer sunglasses, there are many replica versions of all the name brand sunglasses available. Cheap metal sunglasses are made to mimic all the classic styles from square frames to wrap around metal sunglass frames. There is a danger though that these cheap knock-offs will not contain the same quality metal and may be more prone to breaking and bending than their high class originators.
If you think you have encountered a cheap replica, you can often check near the hinges to see if the metallic look is achieved with a coating over pot metal. The extra cost metal sunglass frames will run you is worth it to not have your sunglasses turn your face green in a few days.