Exhaust the hot air, that way you arent using a second AC to cool the entire room, you're just getting rid of the hot air and drawing in cool air from the rest of the house.
If the AC is of portable kind, where you have a hose that expels hot air through the window, it will be a win-win since this also creates a negative pressure, pulling cool air from the rest of the house.
Otherwise if the house is cool, then it's actually any way that's good. The question would be if you want to be able to control the temperature in the room yourself, or want to rely on the temperature of the rest of the house.