ANXX - I usually only travel there between harbours on south of England and Scapa Flow, but I do regular hydrophone checks because I travel across that large grey area of high naval traffic on the east - so I often meet some merchants or convoys there. There are task forces too, but I dont actively hunt them because they are very fast.
AMXX - I prefer this area around Ireland because of main western convoy routes and that large high naval traffic area on the west. Irish sea is also good area for hunting the traffic because of its bottlenecks where you can just listen and wait for merchants to come.