As far as the 2 ships with 3 attack vs 3 ships with 2 attack. I'll let someone who's better with the math then I, deal with that. But Vorpal is right in saying the 2 X's have better offense.
They do.
It's six attack dice either way. Against two attacks the defender rolls defense dice twice. Against three attacks he rolls defense dice three times which, on average, cancels more attack dice.
Two X hit harder than three Z. They die faster, too.