Depends on situation also, if you have a large clump of enemies that need to go down and the group does not think it could dps in time, caster lb would be the way to go. Single target enemies like bosses or a target that needs to go down because of death otherwise, Melee would be best.
Other limit breaks such as Bard does have it's use at times, obviously this was an old post as Bard lb is no longer a heal one, lol