I'll go with AST/SCH as well. A Diurnal AST has sufficient power to fill the WHM's base healing duty, while the SCH has the bulk of the DPS and the emergency utility covered.

AST/WHM feels similar to WHM/WHM in that it's doable, but not the best synergy. AST doesn't bring much of anything that WHM can't do (let's not discuss the current somewhat sad state of the card system...), unless perhaps you go Noct and function basically as a fairy-less SCH without its wealth of emergency CDs. With AST/SCH, at least a Diurnal AST can provide the sort of easy, constant HoT application and on-demand Medica-style healing that a WHM would contribute to compliment the SCH's style.