I was impressed by the weight and feel of the no-lock 642 I bought early last year. It was horrendous to actually shoot though, and I ended up selling it soon thereafter. If S&W made a new variant of the 640 without the lock and with an exposed ejector rod I think I would've been happier with it. The weight differences in terms of actual carry were fairly negligible between the steel and aluminum frames in practice, but the steel was a lot more pleasant to shoot.