I wouldn't run most bulbs on an HQI because of spectrum shift on them, as well as burning them higher than spec. After seeing Radiums on an electronic ballast, considering that's almost the bluest bulb you can get, on the second bluest ballast you can use (only bluer/underdriving would be standard m50? magnetic non-HQI), and I'm okay with how blue it is.
If you only plan to use Radium bulbs, than HQI is the way to go, hands down. If you plan to mix and match bulbs, I would stick with an e-ballast. I know that personally every time I replace my halides, I change to a different color/brand/type of bulb, but my e-ballasts just keep on firing with no problems.