That depends on the meaning or 'durable'. If you mean mechanically durable, the larger, four stroke engines as fitted to tourers can go for many thousands of miles with normal servicing. They are not so high revving as a two stoke and suffer less mechanical wear and overall stresses, although they do have more moving parts. Air cooled motors also have no cooling plumbing to go wrong and leak. If you mean resistant to corrosion in normal all year round use, thats a hard one to answer. Most motorcycles are very poor compared with cars for being able to resist rust and corrosion when used all year round, especially in England where the local authorities coat the road with salt in the winter to combat ice. A few manufacturers are better then others, and you get what you pay for. A little self preparation can also go along way like undersealing metal mudguards and waxoiling hidden metal parts that are not near heat, like seat pans etc. BMW and Honda fare pretty well in quality of finish. I used a BMW K100RT all year round and it was good provided it was rinsed off regulary but try this on a Kawasaki or Yamaha and the bike will almost desolve into a puddle. Ask yourself: why have so many people chosen air cooled BMW boxer engined bikes to do round the world trips?