I was a WHM main in 1.0, and now I'm a SCH main (due to it being what my coil group needed) in 2.0. The way I see it, SCH makes the better tank healer, while WHM makes the better party healer.

The main reasons I consider SCH the better tank healer is being less prone to overhealing (Due to Adloquioum's galvanize effect), the massive bonus Adloquioum gets on a crit, and Lustrate being a better "oh shit" button than Benediction due to a lower cooldown.

In addition, SCH beings a few extra toys to the table like enhanced virus and Sacred Soil that help make things a lot easier. WHM may be the better party healer, but a Sacred Soil before Titan's stomps or during turn 2 enrage strategy does wonders for survivability.

In short, while double WHM may be better than double SCH, I firmly believe that having one of each is the best option of all.