Radiant tubes in a concrete/mud slab vs mounting the tubes under the
sub floor.

I am building a new home and I will be installing radiant heat.
Should I install the tubes on the sub floor then pour concrete/mud
over the tubes or should I install the tubes under the sub floor?
What are the pros & cons of each? Is it worth the extra expense to
pour the concrete? It is not an issue of weight on the floors. The
floors were designed to carry the weight of the concrete. I am
looking for comfort and efficiency once I am using the system.

Barnaby