Like others have said, the 642 and 442 are not just the same gun with a different finish. The 642 uses stainless steel and the 442 uses carbon steel. The stainless will resist rust better which is why I would choose it over the 442. The 640 would be even better if you don't mind the extra weight.
You're talking about the cylinder, right, because they are both aluminum frames. I have never had a Stainless cylinder rust, I guess it is possible. I have a 642, and have to admit both of the finishes on the guns are not the best or thickest.