TBH I tend to make the decision based on target and angle, If I'm end on always magnetic, If I'm near 90 degrees I'll usually go for the impact option, Tho last night stuck 2 impactors into HMS Nelson moored in Gib harbour with little effect, So growing frustrated Stuck a magnetic under her which caused massive secondaries and had her on the bottom in under a min My 2nd Battleship in my current campaign