Weight is the main advantage and I believe ( I dont have the exact science behind me here but.. ) I believe the aluminum by nature has a higher critical speed than steel. Meaning it can spin higher RPM safer without whipping.
As for drag racing the stock steel is likely stronger but much more eveident on an M6.
If you want a great aluminum shaft for a good price get one of these; http://www.exoticperformanceplus.com....html?item=827
Tough as hell and light weight too