Never, Ever take Spirit of REdemption, Ever. Your #1 Goal should be to NOT DIE, EVER. If you are dying frequently enough for it to be viable you need to reevaluate what you are doing to cause yourself to die this much.
I don't totally agree. While, as I said earlier, it's not the most useful talents, it has it's uses in progression when you want to stay alive the longer possible. It will rarely happen but it can make or break a fight and this is not as garbage as people think.
I'm sure we all have those occasions where we wipe at a boss with a few HP left, keeping a few members alive for a few more seconds could mean the boss would be dead. I mean, try it, see how it feels, don't dismiss it straight away

There is one last thing I am questioning. You said that mental strength scales with buffs and gear. Does that include elixirs and distilled wisdom? If it includes ALL buffs then for sure mental strength is ALWAYS better than spiritual healing.
Yes it works on all buffs.
But I don't totally agree that it is always better than Spiritual Healing.
You can think of it this way : having +700-1000 mana is always compensable by using a potion or a mp5 buff if the fight is long enough. Also you will have a pretty good regen at some point and it will make up for some extra mana.
You however cannot compensate +healing by potions.
It is true that the more +hb you have, the less the "base spell" (healing part without healing bonus) is compared to the "bonus spell" (healing part from healing bonus).
But since spells like PoH or FH have a low coefficient (respectively 28,6% & 42,9%), Spiritual Healing does add a low of equivalent healing bonus (eg. 207 healing bonus for FH7). While FH is not the bread & better of a priest, you certainly must be using it quite a lot on trash. And some trashes are hard, so you want strong FHs.
I recently respecced from 26/25 to a classic 21/30 and I haven't noticed the loss of mana. However I did appreciate the healing bonus gained from the talents.