I really don't have a good answer for you. I thought I read that the lightweight driveshaft was also made of slightly thinner material. Could not find the info to back that up. I believe that one is painted white and the other is painted gray ("As deliverd by ..."). This may not be the best way to tell, as a lot of tracks require the driveshaft to be painted white. Is it worth going to a l/w driveshaft? Well, it would create less reciprocating mass.
Sorry I don't have a good answer for you. Hopefully, someone that is "more in the know" will give you the answers.
Steve