The biggest thing to take into consideration with any digital ballast is operating frequency. Unlike magnetic ballast diigital ballasts are not ANSI certified. All magnetic ballast are ANSI certified to operate at 60hz. SO most bulbs are designed to run at 60hz. Since there is no ANSI certification on digital ballasts some may run at 10k hz and some as high as 100k hz, you have to try and find the right bulb for that particular ballast. Under my understanding Next Gen and Quantum have some of the highest operating frequencies of all the digital ballasts. AN makes a low frequency digital ballast and now
EYE Hortilux is making a low frequency digital ballast both utilizing square wave low frequency drive technology.
Digital Ballasts are very efficient in floroecent lighting because its improves the lamps efficiency to run at a higher frequency. This is not the case with digital HID ballasts, thats why I would say the Phantom is better because it operates at a lower frequency and you will probably have less bulb compatibility issues using Phantom ballasts.
Also I would like to comment on the "dimmable" feature on most electronic ballasts on the market. This is done using pulse-width modulation which is pretty much a fancy way of saying chaniging the frequency. This will not save electric because the output wattage remains the same the only thing that changes is the operating frequency. This is a common mis-understanding about dimmable digital ballasts. People think if they turn down the dial they are saving money, but that is far from true.
Hope this helps - GC