Robust bow build - Slayer or Champion?

I don't have a real build yet cause my current one is all over the place with lots of non-bow skills.
What bow build I'll get depends on which Ascendancy I take.

I was leaning towards Champion for the survivability and constant Fortify (20% hit reduction even against Chaos damage) but after looking into some Slayer builds it seems Slayer has high survivability as well with the leeching, so I may want that instead.

Champion also has a a focus on evasion / armor which seem to be not that important later and even has 30% melee damage which is pointless for me.
Looks like Slayer is the more versatile options cause leech works against everything?

Also, is Vaal Pact worth getting for a HP leech Slayer build?

And finally, how much extra mana do I need when I'll use 1 or 2 auras (probably Haste + some Herald)?
I'll get Mana flows for the 40% mana regen by spending only 2 extra points, but should I also get Primal Spirit?