You'll get better tonnage sinking merchies. You should only be targetting warships if it's something special (ie big and important), or you're clearing away a destroyer screen from a tasty convoy to use the deckgun and save some fish. Sinking destroyers and armed trawlers may sting the Tommies a little. Torping a tanker full of avgas, or a troopship full of Yank soldiers ready to bitchslap the fatherland, will hurt them more.
