I'm always a light. We already know the shields are for ARs. And bayonets to me also seem like for ARs only. Lights always have a pistol and so always has a knife. ARs are not available for lights. For these reasons, I will not use both the shield and the bayonet.
If it's a gun with good close range accuracy, I'll use a shield to protect me a bit more. If it's a gun with good accuracy over long ranges, I'll use the bayonet to help me close-range.