The magnetic HQI ballast will run the lamps brighter, but it will also draw more power. Also, they're larger, heavier and run hotter than the electronic ballasts. The electronic ballasts are compact, light and draw less power, but they don't run the lamps as bright. There have been some claims by at least one manufacturer that electronic halide ballasts will give better lamp life and more consistent brightness levels throughout the life of the lamp, but I've not seen any test data to confirm or refute that so far.
Also, some people mistakenly believe that the electronic ballasts are more efficient, but that's not necessarily true. If you look at Sanjay's work, it depends on the specific lamp/ballast combination. Sometimes the magnetic HQIs are drawing a lot more power, but they're producing an even greater increase in light (ie. more light per actual watt consumed).
HTH