The 320 Spindle will do more damage.
The weapon's Light Level determines base damage, which is then modified by some small % based on your overall Light Level.
Think of it like your Spindle has a 320 DPS rating then modified by a 311 ATK rating. It's still going to outclass a 311 Spindle with a 315 ATK rating because your base damage is the far larger factor.
But in any case it's likely to only be 3% or so more damage per shot.
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Cool. Thanks for the detailed explanation.