I run Immortal and know that Hybrid get's more in terms of pure mitigation but the difference is miniscule, to quote an earlier post.
I found in Hybrid I took 18 DPS less, while that doesn't seem like much in a fight that goes 5 minutes it works out to 5,400 less damage taken.
From a healer PoV that's just not a noticeable difference and RNG will have more of an impact, as will any derps by DPS.
I prefer Immortal on snap threat basis and because CB looks a lot better than Impale (not sure about Guardian animations). Resource management is also easier.