"The Slaver Weapon" is a nice episode but, to me, it honestly feels like a generic story with our 3 characters shoehorned into the existing roles. Sulu and Uhura have more screen time but we don't really learn anything new about them either.
"The Pirates of Orion" has an average plot but it has a lot of little character insights along the way. Spock tries to play "superman" again like in "Operation--Annihilate!" but Kirk cuts his duty time in half. McCoy backs Kirk up by claiming it's "Doctor's orders."
McCoy feels the limits of technology upon him again as he tries to cure Spock. It's a nice little nugget of info that supports McCoy's overall distrust of technology that played a big part in his father's death that was shown in ST V.
I give the nod to "Pirates."