Personally, I prefer WHM over SCH in a 4-man DF/PUG group; DF'ed with too many bad SCH leveling LNC, so I'm a bit biased :( It also seems that WHM is a little more forgiving on bad players than SCH is, kinda like how PAL is over WAR from my time leveling WHM.
In an 8-man DF/PUG group, I like one of each. The synchronization between two is great. Titan especially is nice to have both (or, in 3-heal setups, 2 WHM, 1 SCH thanks to the SCH buff not stacking...that really needs to be changed IMO. Haven't done any Coil yet; no groups so far).
Now, that's not to say I will not group with SCH, or think all SCH are bad solo. I know some SCH who are just amazing at their job, and love to run dungeons with them. But that's not DF/PUG...which is the main problem I see. An unknown SCH, from my experience, is more likely to be a sub-par healer than an unknown WHM.
