That's a pretty loaded question because, as Gnu pointed out, your best bet is to have one of each healer for most endgame activities.
Generally, WHM can perform the biggest AoE heal without much prep time while SCH needs to plan a lot more in advance. In return, SCHs are rewarded for planning ahead and the workload is split between you and the fairy. I find it's often easier for me to maintain mana on SCH compared to watching my WHM counterparts...but, at the same time, I can't swiftcast a medica 2 on my group to restore a lot of HP. If I need to AoE heal, I need Eos' rouse'd whispering dawn (which has no base healing component: just a regen) or I need to spam succor which will OOM me fast.
Benediction is nice, yes, but currently certain mechanics favor the SCH ability lustrate over benediction since benediction has such a long CD. While benediction restores all the target's current HP, lustrate just restores a % of the target's max HP. This is ideal for tanks because they stack VIT which increases their max HP.
This means it technically isn't counted as a heal (actually an ability) which means it isn't affected by debuffs that cause targets to take less healing. This is huge on fights like Twintania when your tank is being brought close to death but has a healing debuff on him. It can also be used 3 times in rapid succession: 6 if the SCH is sitting on another set of aetherflows.
But you want advice on ADS I see. And, you seem to take the right path. I'm afraid I have little advice to offer you when it comes to the right path. Is your party composition mainly magic DPS? If so, I understand why you go right. If it's evenly split, though, I would consider trying the left side.
The problem with going right has already been made apparent to you: ADS dishes out a lot of damage. Him having a haste buff doesn't help things either.
For left, our WHM and SCH tend to focus on both tanks. Our SCH uses lustrate almost entirely for the tanks. I believe our WHM will raid heal when necessary but both are capable of performing this task.
What really helps is having a virus rotation, eye for an eye rotation, and keeping track of how many stacks are on a current tank. If your tanks can, tell the healers how many stacks they have on them. This will help your healers prep emergency measures based on # of stacks on a tank.
ADS (left) is also a fight where no one but the tanks should be receiving damage (if you go left). There is minor AoE damage throughout the fight but either healer is capable of capping everyone off when they dip low. Everything is avoidable on the left: piercing laser, ballast, gravity well, etc. If people are getting hit too much by things like piercing laser, then that's not a healer issue.
If you can tell us more about your group composition and give more details on who is dying to what and when, you'll probably receive some better advice.
Edited, Dec 10th 2013 1:17pm by HitomeOfBismarck