Power Boost increases the duration of your control powers (holds, stuns, immobs, knockback, etc) and increases the amount of healing (either self or target heal), defence buff/debuff (Fortitude, Freez Rain, FF bubbles, ...), endurance recovery (from powers like AM), etc.
Primal Mastery would be a great choice for your Ill/rad. I can't tell you which EPP you should go with your Stormie since I find them all quite handy - Primal for PB + FR goodness and end management; Fire for extra aoe damage, Ice for extra control and panic button (hibernate); Psi for mezz protection and decent AoE option; and Earth for +HP or self heal for critical moments, nice PbAoE and good ST ranged attack.
@False Fiction - Virtue / Defiant
Current projects - [Glaciologist - Ill/Cold Troller] [Cloudshaper - Storm/Dark Def] [Harald Wartooth - Elec/Psi Domi]
Am gonna take Primal on my Ill/Rad and its one of the only characters I've known in advance for sure what epic I'll have
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Thanks. So it's as awkward as I thought for a Plant/Storm. And any other than Psi would fit thematically. (Fire would be for Consume more than the AoE - by the standards of Roots, Fireball ain't that great given its recharge and End cost.) So it's a choice between End (Fire/Primal) or panic self-heal (Earth/Ice). I suspect the answer depends on how much my aggro management improves.
And the Ill/Rad choice does seem to be fairly clear, I agree 
*Everything* fits.
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Power Boost increases the duration of your control powers (holds, stuns, immobs, knockback, etc) and increases the amount of healing (either self or target heal), defence buff/debuff (Fortitude, Freez Rain, FF bubbles, ...), endurance recovery (from powers like AM), etc.
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Actually it doesn't affect kback anymore (since it got nerfed with I12) and has never affected +recov (only direct +/- endurance powers like Transference). On the other hand it does affect ToHit (de)buffs.
CoX 50s: <ill/rad> <ice/ice> <fire/kin> <grav/sonic> <ice/storm> <earth/kin> <kin/elec> <cold/psy> <thugs/dark> <fire/dark> <dark/elec> <night widow> <EM/ninj> <mind/icy>
What does the Power Boost ability from the Anciliary Primal Mastery do? IIRC it says +120% to things - but is this durations, magnitudes, or what?
One reason I'd like to know is that I'm debating my anciliaries for when my Plant/Storm and my Ill/Rad get the pools (they are both L29 so it's a way to go). Also, any advice on which to pick?