Ive gone with primal forces, due to power boost, temp invurebility and conserve power. These are all useful powers. But it really is a personal choice!
If you're purely PvE, fire is the one to take, fireball with containment is massively overpowered. If you're planning on PvP, either psi for the mez protection and some rarely resisted damage or primal for power boost.
Given Storm's endurance needs and what you get from Power Boost, I would suggest Primal. I've taken Psi on my illusion troller (love the status protection and psi-resistant shield) and Fire on my fire troller (because kin alows you to boost the damage of Fireball), but still think Primal might be better suited in your case unless you want a PvP build, in which case Psi might be better.
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>
i'd get ice and the first power Hibernate...
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If you're purely PvE, fire is the one to take, fireball with containment is massively overpowered. If you're planning on PvP, either psi for the mez protection and some rarely resisted damage or primal for power boost.
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/em, agree
@Byrne
Debtmeister General of The Plastic Army & Empire
I'd say psionic mastery or primal forces 4 PvP and stone mastery 4 PvE 
@Scorpio EU
I took Fire mastery on my controller simply so i could lay down some extra damage once the enemies are locked down and Consume comes in handy for that extra End boost mid fight
My controller is getting close to 41 and i thought it might be time for me to think about what epic power pool i should take. I am ill/storm and was thinking about either taking the psionic pool as it has a hold res power and a pretty cool aoe, but i am tempted to take primal forces as it has conserve power (which would be very useful) and power boost. I have had a look at the others and they don't appeal to me, so i would like anyones advice on which is the best for me to take.