The biggest difference is the weight, most noticeable if you ever try to carry in your pocket, or on your ankle.
The weight will also be noticeable when you shoot +P ammo. Heavier guns are easier to shoot than light guns. Both guns will be much easier to shoot with rubber grips that cover the metal backstrap. (Metal against thumb bone hurts.) That's another trade-off. Easier to shoot = harder to conceal. Life is full of compromises.
The 442 barrel and cylinder are stainless, just like the 642. It's the frame that's aluminum alloy. Unless you live on a houseboat on the ocean, corrosion is not likely to be an issue. Wipe it off with a Kleenex every once in a while.