Which one should I get, the Smith & Wesson 442 or the 642?

You have the big difference down, carbon steel with a blued finish verse a stainless finish. The stainless should be a better finish, it will resist corrosion better be it from sweat or body oils when handling or carrying. It will also resist rust better if left along for a long time. I'm not trying to run the 442 down. If you like the blued steel look, buy it it's your purchase. I however would get the 642 most every time.