Damage protection is more complicated than soft vs. hard. It’s real engineering.
Say, for example, that I want to stop a bullet from breaking an egg. In example 1, I put a long, straight cuboid of soft foam rubber that stretches all of the way from the mouth of the gun to the egg. I’ve carefully calculated it out - arranging the distance from the gun to the target - so the bullet comes to a gentle stop just inches away from the egg. Success! In example 2, I place a conical shield up against a brick wall, with the egg protected underneath it. The metal deflects the bullet and the force of taking the shot is routed around the egg and spread out through the rim of the shield, into the wall. Success!
There isn’t a hard rule. It’s going to depend on a lot of factors of what your expected impactor is going to look like, how fast it moves, how much you can guess the direction of the blow, how much damage you’re willing to take, how mobile you need to be, etc.
In terms of fat vs muscle, as shielding, I think the primary differentiation would depend on the speed and mass of the impact. You want to bring the mass to a stop over as long a distance as possible, before hitting anything vital. Too soft, though, and you won’t have brought it to a stop and, likewise, too thin and you won’t have brought it to a stop.
Given that you tend to weigh the same, per calorie, regardless of how much muscle/fat you have my suspicion would be that the higher density of muscle is almost exactly equivalent to the thicker but less dense fat in terms of stopping ability. Or, at least, the difference is probably negligible.
That said, the location of fat stores (on men) could say something about the locations most in need of protection. And if both are equivalent in protectiveness then where those extra calories go on the body is probably more important from a defensive standpoint.
But…that said, defense isn’t always the most important thing.
