That’s one of those things where it’s really hard to tell. They all claim different capabilities but the proof is classified (and even somewhat untested). Composite will give you almost no-gap coverage though, and any nearby folks will be happier if you aren’t waltzing around with outward facing explosives all over your tank. But, many Abrams in Iraq added reactive armor all over the less protected sides to help counter RPG’s, so…

I’d mainly say it’s just easier to retrofit an older tank with ERA blocks than it is to design and produce a whole new one with thicker integrated armor.