Kinda depends on the fight, to be honest. Unlike DPS, healing metrics are based heavily on your group's encounter strategies, and how good your fellow raid members are at not taking avoidable damage. A Sorc can pull ridiculous numbers (though not with infinite endurance) if everyone in the group is taking moderate regular damage and is stacking in a nice neat little group. A Sorc is somewhat less powerful at tank healing, particularly burst healing (an area Mercs excel at). Overall, HPS numbers mean a lot less than DPS numbers.
To add to this, a lot of the time HPS isn't even important, but burst healing when it counts. For Commandos/Mercenaries and Scoundrels/Operatives, they tend to be at their best when few targets are taking a lot of heavy damage, compared to Sages/Sorcerers being at their best when several players are taking light to moderate damage. No one metric can really adequately compare them because their fields of expertise differ significantly.