I searched, nothing came up that said "this is the difference" tried in titles only then in the message body. After an hour or more you might find it.
I think I found the answer some time around 2007 or 2008, I don't remember what the actual numerical difference was, or if it was even on this site where I found it, only remember it was enough to prompt me to buy one.
So I weighed my steel drive shaft that I pulled yesterday.
I also weighed my aluminum 1LE WS6 aluminum drive shaft that hasn't seen the light of day since 2011.
The Steel drive shaft weighed in at 15.8 pounds.
The Aluminum drive shaft weighed 12 pounds.
I hear the newer LS1 aluminum drive shaft is even lighter.

 

I run a steel shaft in my race car. The weight saving of a few pounds isn't worth the extra strength gained on a steel shaft. An aftermarket aluminum shaft is better than an OEM one but then the weight would probably be back up to the weight of a steel shaft plus the aluminum shaft is larger than a steel one.