I regularly alternate between series 1 aluminium and series 2 stainless steel. The serious one aluminium is slightly lighter, but hardly noticeable really.
That was one of the turn off's for me with the Sport Watch. It was far to light to me. It gave me the nostalgia that it felt cheap. (Even though it's not) Once I purchased the stainless, I was sold. The Sport was ideal for fitness though.