Renown in game may give you nothing - but the real life U-Boat captain who can claim a cruiser (as well as other sunk merchants from prior engagements) would garner more respect than not sinking the cruiser - but sinking 2 more ships.
Also, destroying a cruiser takes one more 'escort' away from future convoys, making them easier pickings for you and your fellow captains. It takes alot more resources and time to build a cruiser than a transport ship.
Just my humble opinion - I always go for anything bigger than a DD first, and have started to take out escorts before attacking convoys just to make my life easier.
But you're the captain of your boat - YOU do what you like
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