like Ian said, the Alum driveshaft does in fact have a critical limit on its fatigue life. Unlike steel, which if cyclically loaded up to a certain point, can go on forever without failure, Al will always have a "cycles to failure" limit. Keep in mind that i have no idea what this would be as it is based on the alloy used, the quality of manufacturing, and the shape of the shaft. Personally, my car will be driven quite a few miles once it gets put back together so I'm going to opt for the steel shaft. If I was going to be building a drag/show/weekend car, I'd likely go for the Al. YMMV.