Both are terrific little revolvers. Action is the same, barring action work of some sort. When you first get it, fill the cylinder with snap caps and do a couple thousand dry fires DA and SA too if you run with the 638. Then see where you are and if you might want a spring kit or a professional trigger job.
I chose the SW638. I also own an old Taurus 651SH2 titanium Protector which has the same configuration. Both have several thousand dry-fires under their belts, a spring kit from Wolff, and Crimson Trace minimalist hard laser grips. SW638 also had "The Lock" removed and replaced with "The Plug." Taurus 651SH2 with laser grip is my first go-to HD handgun(1). It is light, so does not dag down PJ pants pockets, has the laser for low light, and gives me an advantage vs someone who might try a disarm. SW638 is my "almost always" gun. If I can not manage a holster for whatever reason, in the pocket it goes, then into the truck console when visiting restricted areas.
I find the SA option useful and have hit targets at some extended ranges with its aid. Once you know your gun and load and execute the fundamentals, getting good hits on targets a ways out there is do-able. J-frames are much more than just "belly guns" if you give them a chance and a SA trigger allows one to wring out all the performance. So much so that if I am hunting and armed with a rifle, my sidearm is my SW638 stoked with hard cast wadcutters. No big bears nearby and that .38spl hard cast wadcutter penetrates like crazy.
Also, the flip side of "dust bunnies get IN easier" relative to 642, is "dust bunnies get OUT easier" relative to 642. A snort of compressed air or a hose down with solvent can traverse through the lock work. without having to remove the side plate.
If you are on the fence, but can spend a bit more, S&W produces some 642 or 442 J-frames that have a trigger job from the factory. That is worth something.
Good luck. Both are good choices, so no worries either way.
(1) 1911s, L-frames, and shotguns also at the ready.