Of the two, I would use the second one. I think it’s better to have the magazine attached to a bone of the player character. Because if you change animation of reloading, you’ll only have to change the character animation, and not the gun and magazine’s animation as well.
There are other options as well. You could have two places where the magazine mesh would be attached, and just toggle its visibility. Magazines would be two separate meshes that are always connected: one to the gun mesh and the other to the character hand bone. When the character takes hold of, or comes into a certain frame of animation, the gun-magazine would become invisible. At that same moment, have the hand-magazine in the player’s have become visible. Reverse the process for when the magazine is returned.
I’m sorry but I can’t offer any insight into performance aspects of the approaches.