From Lumatek's website.

why choose electronic ballasts?
More lumen output - up to 30% more lumens output than some old style magnetic ballasts ...More light equals more yield. We urge you to test this yourself with a good quality light meter. Be sure to also check how much power the ballast is using with an ammeter as many so called 600w magnetic ballasts now draw up to 750w as standard to try to keep up with Lumatekā??s lumen output.
More lumens generated per watt of electricity consumed - Lumatek ballast have increased efficacy over magnetic ballasts. They lose fewer watts from the power supply to the lamp. This a power factor of 95%+. The high frequency output results in low power loss at the cathodes in addition to better overall power management.
Stable light output - excellent for Pharmaceutical-agriculture, laboratories and other uses where precise regularized output is essential.
Fast start-up - It reaches full brightness in under one minute; magnetic ballasts typically take about twenty minutes.
Completely silent
Small compact design - 600watt 120v ballast weighs less than 4.5 kilos compared to almost 20 kilos for magnetic ballasts.
Produce less heat
Cut-off circuitry - cut off when a short is detected. For ultimate safety.
Longer bulb life - Lumen output loss over time, with the correct lamp, is dramatically less than with magnetic ballasts.
Fully interchangeable - Lumatek ballasts can light both HPS and MH bulbs (excluding the 750w which is HPS only).

So I'd just need to purchase an MH and an HPS bulb. However I've herd many times you can produce great plants with just using the HPS for the entire grow.