OT: Technically, it was the same rocket, just with different warheads.

A solid warhead was intended to penetrate tanks, while a HE warhead was developed for use against subs. However, hitting a tank dead on with a rocket is hard, while hitting a sub with one is much easier. In addition, the 3″ chunk of fast-moving steel *did* penetrate right through u-boat hulls, even if it had to go through several feet of water first. And frankly, a few 3″ holes in a u-boat would sink it; the kill rate with RP against diving subs was much higher than with depth charges.

Meanwhile, while the HE warhead wouldn’t necessarily kill a tank outside of the same lucky shot, but it would devastate lighter targets.