I know your 642 is a snubby. But I'm curious what the barrel length of your 686 is.
For the 642, Mike Irwin, engineer88, and I think alike: It's the Speer 135g GDHP SB +P. Great short barrel round. In fact, it's my home protection round in a 686+ w/ 3" barrel.
The best personal defense round is the one you're comfortable shooting.
Getting to bullet weights: all things being equal, heavier is better. But guess what? . . . all things are
never equal. Ultimately you want penetration. It's mass (weight) that penetrates. Physics 101. Without proper penetration, you've got nothing. Expansion is important; but only
after penetration. Makes sense? I think sometimes we shooters can't see the forest for the trees. We tend to overthink and get into too much minutiae.
So a heavier bullet is better. Right? Not so fast. . .
There's also overpenetration - especially in a public setting. Basic Firearm Rule #3: Be certain of your target
and what is behind it. And at low velocities, a heavy bullet may not expand enough to deliver its energy into its intended target and pass on through to do collateral damage to that dreaded place called lawsuit city. This is where lighter bullets start to look attractive
I could go on. This is a huge subject. But I'm typing this while at work. Somebody relieve me, and take over please