Which error is more prevalent in the world—legalism or license?
Globally speaking, license is more common than legalism. I say that because we are wired to sin by doing what we want to do. The only time we become legalists is when that inveterate, self-centered desire bumps up against rules and religion. Then we say, "Well, to save my skin I better start performing this, that, or the other." That's when license takes the form of legalism, because we're now doing our own self-designed salvation.
There are, of course, many legalists within the major religions of the world. But probably more common, more human, is that we get our pleasure the way we want. Legalists have just bound that pursuit of pleasure up with rules and religion so that they can save their own skins and keep God off their back. But both legalism and license are huge errors, and both are prevalent.
And both are deadly. I don't want to communicate that licentiousness is the worse sin. In fact, Jesus seems to have the hardest words for Pharisees, not prostitutes. That's because the Pharisees knew so much, pretended so much, and were so rotten on the inside. Their legalism and strivings, both religiously and morally, didn't make the Pharisees any less evil to Jesus. It made them more evil.
Paul also speaks forcefully against legalism, doesn't he?
Yes, he does. Those who are trying to be completed in the flesh, they don't understand where they've come from, they don't understand the Holy Spirit, and they nullify the cross (Galatians 3:3). If we try to begin by trusting Jesus and then move on to rule-keeping as a way to be right with God, we will undermine the very meaning of grace and the meaning of the cross (Galatians 5:4). We must get saved, stay saved, and do the will of God while we are saved, all by faith. Doing never replaces believing in the Christian life. Rather, we are to only grow in our child-like dependence upon God.