I've always installed the Pex tubing for radiant heating within the concrete slab. This allows the concrete to be a thermal mass storage device and will radiate the stored heat back into the space. Typically we use this product as it addresses two issues, insulation under the slab, and a simply way of running your tubing. http://www.barrett-inc.com/creteheat.php
check with your building department to verify the minimum insulation R-value required to be installed under the slab.
everyone has their preferred way of doing things, and as long as it provides the same results whos to say which is better .....
Hope this helps!