Hmmm, it all depends on the situation
If I have any of the following: bad angle, tanker, serious capital ship, very shallow draft or badly shaped hull (e.g. small tankers almost not vertical sides on those) I use magnetics
If the following applies: BEFORE 1941, good angle, rough water, shallow water, passenger/cargo, cruiser target or if I know the sweet spot (I'm nah tellin' nuh-uh) use impacts
suffice to say that experience counts, I wouldn't even know have many eels I've fired since I started playing this game, but some of the best ones stick in your memory and become something of a signature shot
for example, medium tankers, shot at AOB 50 or 60, magnetic depth 11,5 meters, fast steamer, range 2,5 km, aimed for the far side of the bridge, sinks them with one eel within minutes after hitting 3 times out of 4
useful info if it's your last eel