I have had great results with magnetics so far(only had the game 2 months). After reading posts and carefully matching my running depths, they make killing an off-angle target much more predictable.
Rather than using an 8 metre standard depth, I use the draft reading from the target recognition manual, then add a half metre or so depending on weather. 15 metre/sec wind makes for waves that a mag torp just doesn't like. I have gotten almost every mag malfunction I have had so far in heavy seas. Nothing draws an escort like a premature detonation. The closer to the keel the torp explodes, the better the chance for a one-hit kill in my limited experience.
What I like best about magnetics is the ability to shoot under the tiny Small Merchant blocking the C3 in a convoy. The added benefit is the escorts look in the wrong place when this happens.
I use full realism(except for external camera), so there is no targeting a certain section of a ship. I feel satisfied if it hits at all.
Impact is the way to go if I can get a 80-100 degree shot. I run them as deep as possible without striking the curve of the hull. Look at the keel depth in the manual, then stare at the underwater part of the hull in external view, and get a feel for how deep you can run an impact trigger torp. the deeper it goes off the more stress on the keel itself(or so it seems).
Now as far as electric versus steam, I have found that using electrics for the farther targets in a convoy and then firing fast steams at the near targets works well. This keeps you covert longer, as the steam torps will have a very short runing time. The convoy doesn't take any action until the 1st torp explodes, and I try to make sure that both far and near targets get hit at the same time.
I have read items that advise using electrics on escorts to help stay covert in case of a miss or dud. I take a more aggressive/confident outlook. If he dies from the hit, it won't matter if he saw it coming, and a steam is FAST. Hitting an escort safely from 2000 metres seems better than hoping he won't turn before my electric can chug over his way. I have been detected trying to get close enough for a high-probability electric shot.
Electric Magnetic - Killing escorts that are actively hunting you at close range. Killing further convoy targets.
Electric Impact - Rough weather convoy targets. Escorts don't even know where to start looking and sonar is degraded in high seas, so you can keep attacking at leisure.
Steam Magnetic - Nearest convoy and any single target.
Steam Impact - Escort killers, and rough weather convoy targets during attacks at night.
Just some observations so far.