escorts have lower renown value than merchants.
as many have said on this forum before - your mission is to sink merchants carrying vital war materials... without fuel and oil they cannot sail the escorts anyhow.
Dont worry about escorts so early in the war... sink a couple of ships and dive deep as you dare, run ahead slow and rig for silent running and creep away.
generally the larger the merchant ship the more renown she is worth, first chance you get - upgrade to a type VII boat... longer range, more torpedoes, deck gun, you have more options this way and you dont have to be so selective about your targets.
good luck out there!
edit: someone a while back posted a list of ships and their renown value, i dont recall where, but if you just go for heavy merchants you cant go wrong.