If the floor finish will be hardwood floor, is it more preferable to locate radiant heat in the concrete slab, or in a batten cavity directly below hardwood floor?

We need a better description of the stack up.

If the finish floor is in direct contact with the underside of the finish floor and the concrete isn't too thick (with an unmanageable amount of thermal mass) putting it in the concrete is the way to go.

If there is an air gap between the finish floor & concrete, (say with a subfloor resting on sleepers above a slab) it's better to put it somewhere else. Ideally it would be above the subfloor with just the finished wood above it, which would take the lowest water temps and provide the most responsiveness. (WarmBoard etc works fine.)

It is the ratio of the R-value below the PEX divided by the R-value below the PEX that determines the performance efficiency. So the less R-value above the PEX and the more R-value below the PEX is the design objective. A hardwood floor adds significant insulation between the PEX and the living space to be heated. There’s nothing wrong with having a hardwood floor as long as you willing to add corresponding more insulation below the PEX or accept the significant performance loss.