The fractal design Celsius S36 is the best of the plug-n-play aios. The Swiftech H320 X2 is the best if the expandable aios. While the S36 has expansion capability, I personally wouldn't on that asetek pump, they really aren't designed for that kind of volume/pressure. With user available fittings, a chance to change hoses or add fluid is the only real bonus.
The Swiftech has a much stronger and more reliable pump, so expansion is a definite ability.
All kinda depends on you application as to which of the 2 is better suited.