My first choice would be to get both, the 340 for pocket carry and occasional practice and the 640 for belt carry and regular practice.

If limited to one and I were going to pocket carry it most of the time, I'd go with the 340 for the lighter weight.

The 640 can be pocket carried, but the heavier weight can be an issue. You could always take your pants to a tailor and have them lenghten and reinforce your pants pockets to better handle the extra weight.

I carry IWB, so if I were making the choice I'd pick the 640 to make it easier to shoot and practice with.

I don't consider night sights essential on a defensive gun, so that's not a big deal to me. I also don't care for moon clips so that option is also moot.

Regarding practice, even if you intend to carry .357 Magnum loads, you don't have to do all of your practice with them. I carry a 642 loaded with Speer SB-GDHP +P. I'd say probably 2/3 to 3/4 of the rounds I fire are standard pressure .38 Special loads and the remainder +P. Such an approach can mitigate any recoil-related issues with practicing.