Hey welcome aboard!
I agree that Impact torpedoes are preferable as they tend to be a little more reliable, but for them to work properly you have to be as close to perpendicular to your target as possible, to keep them from simply deflecting off the hull and alerting everyone to your presence.
If you spot a juicy target but are unable to get yourself in the ideal AOB=90 position, then it's worth it to go with magnetic: As your torpedo will be coming under the vessel at an angle, it will spend more time under the hull, giving it more time to detect the ship's magnetic field and detonate.
hope this helps, and listen to what your fellow Kaleuns have to say, the experienced ones have learned their lessons through countless sinkings and ships sunk!!
High Voltage