I own an original Galaxy S phone....Plastic feels cheap, whether it is lighter than metal or not. I don't like paying $200 for a plastic body like I had on my 6-year old Nokia.
In fact, the only reason I got my specific Galaxy phone(the Captivate) is because it had an aluminum back plate. Had it been all plastic, I would have gotten an iPhone.
I just weighed my 4s at 146.1 grams and my friends SGS2 at 137.8gms