M&P 340 vs 640, very tough choice. I've owned the M&P 340 a couple times (currently have one barely used) and have considered the 640 to the point of even putting one hold, but so far haven't purchased. These two are by far the only two modern J frames I'd buy (and the no-lock versions of both), and I've spent many many hours comparing the two, and still haven't completely come to a final decision. Close but not final.
As you know by now the answer depends on your requirements, modes of carry, philosophies, etc., but my opinions (I stress
opinions):
In short, forget aesthetics, moon clips, glowing sights, etc. It all comes down to two factors:
1) Is pocket carry a must?
2) Are you comfortable shooting .357 magnums from a J frame?
If you must have pocket carry option, and especially if you pocket carry frequently, the 340 wins, no question. I find any steel J-frame too heavy in a pocket: it swings the pants about when you walk, it's bulky, and is hard to draw quickly and cleanly. I don't even pocket carry the 340 for the last two reasons. If you belt carry or IWB carry, either is fine in terms of weight, but if pocket carry is a requirement, go for the 340. I also don't pocket carry any gun because when you sit down, it's pointing down range, not down.
In terms of shooting comfort, .357 magnum is unpleasant in any J-frame. I've done it from a 340, 649, 60, etc. using up to 158 gr bullets in downloaded handloads and factory full power loads. I'm a medium build with medium sized hands. Unpleasant much more so in the 340. Steel 640 helps here. Still not pleasant but far less unpleasant than it is in the 340. If I was just carrying SHTF ammo I wouldn't mind (so I tell myself), but if you're doing any range time with magnums, even downloaded lighter loads, the 640 will be less punishing.
The most important difference between these two is the weight. Far more important than size, sights, barrel length, aesthetics, moon clips, etc.
I believe you've lowered it down to the best two modern J frames. I'd do whatever you can do to own both, and spend some time getting to know them, and keep the best, sell the other. Or not
Lastly, if you're willing to buy used, I would also consider a 640-1 (anything after the 640 no-dash will handle .357 magnum). You don't get moon clips or glowing sights, but you get a quality pre-lock piece, without the "Pro Series" script and fluted barrel (I'm not a fan of either). Again, pocket carry, maybe not.
Current fave and daily carry here is a humpback 49, looking for a 49-2 or 649-2, have a 340 posted FS but now talking myself out of it LOL, looking for a 640 and considering a 640 Pro. See? Not easy.