Both Negotiator and Malevolence will be farmed with GET2.
Malevolence will almost certainly be a bit better than Negotiator but not much. Why not much? Because they just released Negotiator not long ago and many people don't even have it yet. They don't want to chase something out of the META before it even gets 7*. So the 2 will be roughly equal as far as arena goes.
Now CG made a mistake. They should have released the cap ships with their respective sides instead of how they did. If they had released Malevolence with dark side TB, it could/would be required for missions. But they didn't they released Negotiator with it, and you can bet your bottom dollar it will be required in light side TB.
So, logically thinking, Negotiator is almost certain to be the cap ship that should be farmed first as they should be roughly equal in arena prowess, but Neg will be required for things and Mal will not.