I didn't feel "safe" on the 550s. The rear spokes would creak and snap when climbing and they "felt" (to me) kind of weak and flexy (and I only weigh about 175lbs). I don't like having the risk of a breaking spoke on a low spoke count wheel on a ride as they do tend to be out in the middle of nowhere and back. So, I bought the Aksiums and am very happy with them despite the few added extra grams of weight. Basically, I just wanted a wheel that wouldn't leave me stranded out in the middle of that nowhere and the Aksiums fit the bill perfectly...at least for me.