Sebazy has summarized everything quite nicely, IMHO, BY THEMSELVES, WHM will be harder to play compared to SCH, mainly due to mana management.

But with the existence of bards, semi-competent healers can just go nuts with their MP, and yell at the bard for Mage's Ballad when they run dry, thus completely annihilating the challenge of Mana Managing as a WHM.

Thus as there's tons of bards running around (9/10 DF for 8-mans will have a bard for me), the difficulty of playing a WHM is somewhat diminished.