Last Prism without question.
It can pierce infinitely and its aim animation will catch up to where cursors are quicker compared to the Lunar Flare. This is because the Lunar Flare has a delayed attack, the magic dealing sprites must first fall from random points in the air and move towards where targeted. Aside from response time the Lunar Flare's projectiles do not pierce mobs, they collide, resulting in hitting fewer mobs per second.
Perhaps there are people who find it useful in controlled setting but in general usage the Prism is certainly more practical since players most commonly fight in wide, unobstructed places.