well in a theoretically having the radiator outside will get you lower temps because you'll need more tubing, which decreasing your water flow. With the radiator inside your case, the idea is to get fresh air from outside your case. In many cases, the radiator or fan is not completly sealed to take air from only the outside of the case, and this will result in air already inside the case, which which is more like to be warmer then outside air, to flow through the radiator.
So in essences, if you can completly seal up your rad/fan and get outside air through only, you'll get better temps.