Strategically the tankers are a more important target. As BBs were superceded by CVs they no longer represent a worthwhile investment in time/resources trying to sink them, they can easily be bye-passed. Certainly they are a potent weapon in a surface action, but the CV battles were the ones that had real effect on the war. For example, if USN had sunk 4 BBs at midway it would have been a major loss for the IJN but it would not have won the battle for the US.
Tankers were in short supply and as Japan had no natural oil supplies, the loss of the tanker fleet would fataly cripple the Japanese war effort.
If there was a CV though that would automatically be the priority, damage it at all costs!
