It seems to me it comes down to how well you can control your situation. Soloing, if you can pretty much expect to always be killing something or medding, I think you're right that survivability is where it's at.
Grouping, though, I think it is especially important for healers (not so much INT casters) to have a nice, big mana pool. The reasons being that 1) in a good group you'll be able to spend all your time healing or medding, there should be no standing around, and so the higher your mana can get while medding, the better-off you'll be for those 2) inevitable overpulls.
My 2cp
|