the speakers set at 16 ohms will be easier on the amp than the 4 ohm setting. If you want to play LOUD, use the 16 ohm setting. The amp will be in a safer region than trying to play loud into a 4 ohm load.
Lower impedance is closer to a dead short. higher is a little more resistance. For your needs (your amp runs hot anyway), higher is better.
Lower impedance is closer to a dead short. higher is a little more resistance. For your needs (your amp runs hot anyway), higher is better.