Factory Functions & Singleton Pattern

What key difference exists between these two approaches?
function Circle(radius) {
  this.radius = radius;
  this.area = function() {
    return Math.PI * this.radius * this.radius;
  };
}

function createCircle(radius) {
  return {
    radius,
    area: function() {
      return Math.PI * radius * radius;
    }
  };
}
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