I am deploying and can earn an ARCOM or a Joint Commendation Medal (which I already have) on this deployment. Which one should I take?

why is that, sir?

To get recognition from your own service is an honor but normal. To have a chance to be recognized with joint award is more unusual. To be recognizing with a decoration from another branch is even more unusual.

Capt (Join to see) - I would agree with LTC (Join to see) . You'll likely not have an opportunity for an ARCOM again. Same reason I would take the AF COMMENDATION Medal if the roles were reversed. If you have the JCOM already, all the more reason.