JavaScript: Where to define properties upon Constructor.prototype?

Question

I'm accustom to work with constructor-functions this way:

function Computer(ram) {
  this.ram = ram;
}

// Defining a property of the prototype-
// object OUTSIDE of the constructur-
// function.
Computer.prototype.getRam = function() {
  return this.ram + ' MB';
}

I define and initialize values within the constructor. Methods are attached upon the prototype-object outside the constructor-function.

Yesterday I've seen these technique:

function Car(maxSpeed) {
  this.maxSpeed = maxSpeed;
  // Defining a property of the prototype-
  // object WITHIN the constructur-
  // function.
  Car.prototype.getMaxSpeed = function() {
    return this.maxSpeed + ' km/h';
  }
}

The method is defined within the constructor-function => Without creating at least one object the method doesn't exist.

What I always consider a bit weird concerning the first approach is that someone can call the method upon the constructor-function / -object. Then all properties which use the this-keyword return undefined. With the second technique an error is thrown.

What are the advantages and disadvantages of both techniques?

Which one should I prefer / avoid and for what reasons?