Both are great weapons, what would you prefer? I have a hard time choosing.

Mjolnir. If I'm worthy, I'd possess the power of Thor.

Mjolnir by far! Assuming I'm worthy and get thor's power

Cap shield.

Mijonlir

Cap shield is cool but you'd have to learn how to throw it properly, and would have to have great reflexes to be able to block things. I also don't think it would come in handy often. I'd pick mjolnir.

Mjolnir. Let Midgard's enemies tremble.

Mjolnir easily.

America Harold, America! Hit people with the Shield of Justice!!!!!!!!

Assuming I'm worthy, Mjolnir. Cap's shield really requires someone of around super solder ability to use it properly. No way in hell I have the skill to make those crazy ricochet tosses that Cap does.

On the other hand, Mjolnir comes with all the power of Thor, which means it comes with the physical ability to use it. Although it would be a pain to make sure it isn't out of my hand for more than 3 seconds.