I have used both, and prefer the 640. With mild .357 loads it is not bad at all. I use Winchester 110g .357's. If an inexpierenced shooter uses it, I load 158g +P lead SWHP. Remingtons are what I like best, but I have some WW and Federal.
The 640 is a little heavier carried in the pocket. My aluminum 642 cracked at the frame. With the all steel 640, I dont worry about cracked frames.
The same holsters work for both.
I suppose if I still had a 642, I would use it. I sold the new replacment 642 S&W sent me, because of the lock. Now that the no-lock is available, if I found one at the right price? Maybe.
One thing you need to remember, what other people think is not as important as your own personel experience. It doesnt matter what I or anyone else says, you must do what is right for yourself. Either gun is a good choice. The 642 without a lock is probably easier to find than a 640 without a lock. If locks dont matter to you, dont sweat the small stuff.