I'm an electrician so this is right up my alley.
Magnetic ballasts are the old style of ballast. They use a coil and iron core to convert one voltage to another, a transformer really. They contain an internal oil coolant, in older cases pcbs. They get way hotter and are way less efficient than electronic ballasts. Electronic ballasts have a capacitor and uses other forms of electronics to give it a better power factor. The light from a magnetic ballast is less usable than electronic due to the flicker they cause.
I could go way more into detail but long story short, don't buy magnetic. No one uses it or buys is anymore as it's way less efficient (power factor), way hotter, and just plain bad for the environment and when they start leaking and get all over your skin, not the best for you either.
Induction lighting, as another poster asked about, is way more efficient than both styles of ballast. They are sopposed to create a much more usable light for plants as they have full spectrum grow lights that you use from start to finish. I have personally never seen them, or their regular high bay counterpart. They are super expensive in comparison to other styles an no one is buying them. Makes you wonder. These became the talk of the trade about 3 years ago, and just seemed way too good to be true. They must have been since everyone is still using HID or fluros for high bays, and fluro and LED for buildings.
I'm not sure of the cost now, but when I was pricing a few out for a job they were $600 for a 400w ballast. Of course this would not be a remote ballast as it's unnecessary for commercial applications.
I would stick with the proven Electronic Ballast, LED or Fluro. Electronic being your best HID option.
