I own both the standard S&W 642 and the Ruger SP-101 (DA/SA with 2-1/4" barrel in .357 Magnum).
My question is, why do you want to trade in your 642? Is it to have a more powerful "pocket" revolver? Because in size, weight, and material the j-frame Airweights and the SP are not even close. The SP is not a pocket gun in my opinion, and the 642 is the quintessential pocket gun.
Or do you just want to step up to a .357 Magnum for all the other obvious reasons? If this is the case, I can't say enough about the SP-101. It's small enough to be a good belt gun with the right belt and holster, but still hefty and solid enough to shoot a constant diet of the nastiest .357 loads.
If you are strictly talking about using this revolver for concealed carry for defensive purposes against human targets, I'm not sure there are enough benefits of a .357 vs. .38 Special +P out of a sub-3" barrel. When you start talking about heavy hard cast lead bullets for "trail" duty, that's where I believe the SP-101 really shines against S&W Airweight j-frames.