The primary reason I do shell over guard is that shell is something I can activate reliably, which I need to get out as many heals as possible. Even though mana spring is an awesome skill, it is rare that I use Clovis' power consecutively since I like to keep him as healthy as possible.
Now in deciding auto potion or antidote, I flat out give all my burst healers this tree(Everard, Clovis, Amadok) because it allows them to operate more independently against tough damage types like elemental and poison. Since they can only heal others or each other, these 3 need to be as resilient to damage as possible.
I find the auto potion route is often more geared towards those warriors who cannot afford the offensive trade off or mana consumption of a healer, this of course, is not a problem that a healer shares.. Therefore the antidote-->neutralize pathway is the most effective at allowing clovis to do his magic.