Erik Killmonger is Black Panther’s challenger to the Wakandan throne, and that is his main preoccupation. He is also interested in spreading Wakandan power throughout the world and using it to help people in need.
That might sound good, but Killmonger is brutal, bloodthirsty, and quite self-absorbed. He even tried to make it so no one could become a Black Panther after him. Overall, given his preoccupation with Wakanda and emphasis on up-front fighting, he would make a poor fit for Black Widow. Stealth and spy stuff are not his forte.
James Rhodey is War Machine, an alternate Iron Man who makes good use of all kinds of powerful guns on his suit. He can fly around and perform melee attacks as well, and he is serious and disciplined in battle.
However, War Machine is rather loud during a mission with all those firearms, and he’s not a tech genius or as resourceful as Tony Stark. Black Widow can’t ask him to hack into an enemy’s mainframe or decipher ultra-advanced science, for example. She’d need someone else to handle it.
Black Widow has already gotten a taste for Captain America as a mission partner. They made a fine, if slightly awkward, team during Captain America: The Winter Soldier, and they complimented each other fairly well.