[Rob Stehlik]

I would just switch to a steel shaft and call it done. I don't think the minimal weight savings you get from aluminum shafts is worth the risk. We have used aluminum shafts in the past, and while we didn't see any catastrophic failures like you show here, we bent a few of them.
In response to your original question, usually snap rings are more reliable than shaft collars, but they do weaken the shaft. You want to avoid transmitting torque past a snap ring groove if possible. Generally I like to use snap rings on the ends of the shaft to keep things on, and spacers in the middle of the shaft where it will see torque loading.