NiMh with a high mAh rating would be better over standard batteries. Don't worry about the voltage when comparing rechargables to non-rechargable. I've used the same set of 4 rechargable AA batteries for 2 weeks with little to no change in performance. That would not happen if I was using a non-rechargable battery.
I use PowerEx as well. Buy 16 of them for $60 or so, and use them all year. I use an sb910 on both bodies, and recharge after the weekend is over. I never have to worry about them going dead and I usually take over 2200 shots per weekend at roughly 1/8 power. Next year, I'll buy 16 more for the flashes, and put this year's 16 into remote controls, clocks, and other household items... although I'm sure you could get 2-3 years of flash use if you wanted to be frugal.
Used to have a quantum when I was running my flash real hard... now that I've turned the power down, and let the ISO do the work, there's no point in using a quantum, so it just depends on how hard you run your flash as to whether a quantum is helpful.