Kabzy is right on one point, SCH has the best sustained DPS of all three heal
Whm got the best burst, but maintaining it kill your mana.
And AST... Well... Medium dps and not so good mana efficiency in DPS.

On sephirot EX there is not so much DPS windows for heal (color phase in p3 and the whole P1 for the off-heal if the main heal can solo it... Like diurnal AST)
Otherwise you need a shit ton of heal during that fight XD