Well,
Here are the 640-1 Pro Series and the 340 PD side by side. Sorry for the iPad photo, but I'm here at the cabin, and lighting isn't too good. I have some other choices, like the M&P 340 as well, but these two represent most of my carry recently, and I think you are looking at something similar.
When I pocket carry, which is how I mostly carry, I use a Galco horsehide pocket holster. Here is the 640-1 in the holster:
And here is the 340 PD in the same holster:
As you can see, they have exactly the same profile in the pocket. The difference is in the weight. The 340 PD weighs a little over 11 oz., and the 640 weighs a little over 23 oz. twice as much weight, not counting the ammunition and holster, which would be the same for each.
When I am pocket carrying in Levi jeans (and you have to be careful which "number" you get, as my old "505"s used to have deep pockets, but they don't anymore, and I went to, I think, "550"s, which had, the last time I bought, deep pockets). Anyway, when pocket carrying in jeans, I don't notice the difference in the weight. But when carrying in a suit trouser pocket, I find the heavier gun does not carry well.
Personally, I'm glad I have several choices, and enjoy switching around. Of course, I spend a lot of time shooting all of my guns, and feel confident in my skills with pretty much any of them.
One note on the moon clips. I keep my 640 loaded with a moon clip, because it will ensure positive ejection of the empties. I usually carry spare ammo on a speed strip. Moon clips are nice, but they are not very sturdy, and are easily bent. This will not happen when used as I use them, but I would not be confident in trying to carry spare ammo in one. But having the capability in no way detracts from the versatility of the gun, it only enhances it.
As a poster noted above "buy both of them!"
I hope that we have given you some help, but ultimately, the choice is yours. Either of these guns, or the M&P 340 would be a fine firearm, and one that you can stake your life on... If you master it!!! Someone, maybe the "Master", Jeff Cooper once said: "owning a handgun no more makes you a great shot than owning a piano makes you a musician!, You must master either one to be proficient!"
Best Regards, Les