Great question, honestly.
Probably Cousins. I think he's a top 10 player. Could be top 5. Hell, he could compete for MVP some years. He's a monster. LeBron would truly unlock his ceiling. He's also shown ability to be a defensive anchor.
I also don't trust that George isn't secretly set on being the guy to make the Lakers relevant again. Don't trust him. Don't like the way he did the Pacers.
Though all else equal...if you are adding one guy halfway into the season that gives you the best chance to beat GS....that might be George. Cousins would be with an extra thought towards the following season and beyond.
Can I cheat?
I'd trade the Nets pick for Cousins and then call up Presti and trade Love for George.(locked up until 2019, Westbrook's good friend, better fit next to Russ because he is willing to sacrifice..also moves Melo to the 3 where he's better offensively...bad defensively no matter what...plus George is leaving OKC in the summer because they stink so it's the best offer he'll get...)
IT/Korver/George/Bron/Cousins (bye bye GS)
Wade/JR/Crowder/Green/Frye bench bunch(bye bye GS)