Assuming that Superman is fully aware of what they can do, and has to chose one of these weapons, which one do you think that he will chose, and which one would amplify his powers the most?
It was stated many times that his will is unbreakable and one of the strongest of all. Outside of Hal, I don't think anyone can claim their will is stronger. Evidently, he has recently come across Parallax and he was able to become a host until Superman allowed it ,
If Superman is full aware of the gems capabilities and he is able to harness that power, lol, that would be his best choice. Punching through dimension, the versatility of the gem and complete invulnerability while at the same time being completely unstoppable.
But he can still be bfred. Superman doesn’t need to be more invulnerable; he can use speed to avoid attacks. He can block attacks with spinning Mjolnir. He can create force fields and absorb energy.
With Mjolnir, he can bfr threats and get back to the battlefield if he is bfred.
Also, he can remote control Mjolnir, creating another attacker, while he operates himself on the opponent. And finally, Mjolnir would tremendously increase his striking power. Just look at the feat in the crossover when he borrowed Mjolnir.
You have to weigh that against how often Superman is going to be encountering magic.
__________________ Chicken Boo, what's the matter with you? You don't act like the other chickens do. You wear a disguise to look like human guys, but you're not a man you're a Chicken Boo.