To answer the OP's original question, I don't know. My guess is the 640 Pro and the 640-3 are essentially the same in quality and performance.
I've got a 640-3, with a few additions. I added a XS Big Dot front sight and had TK Custom moonclip the cylinder. I also added an old-style thumbpiece and neutered the lock. Those mods bring my investment over the cost of the 640 Pro, but they were done over time to cushion the blow of spending the money. I still don't have the excellent rear sight of the Pro, which would involve work by a good gunsmith and drive my investment for the standard model beyond the price of a Pro. That sight is the nicest thing about the Pro, in my opinion; the fluting is fine but essentially useless.
According to the S&W web page, the 640 Pro weights .3 ounces more than the 640-3, which I don't understand, unless that is attributed to the extra weight of the front and rear sights on the Pro. In which case maybe the fluting is a good thing, if it helps keep the weight from going over more than 22.4 ounces.
Overall, my 640-3 is an excellent revolver that has about 500 rounds through it flawlessly and handles .38 +P very nicely and can deal with .357 Magnum when it has to (my carry rounds are .38 +P). My preferred practice ammo is the 148 Grain LHBWC, whether store bought or hand-loaded, because it is a mild round - a friend to the J Frame shooter.