For power consumption, electronic ballast, pretty much any of the reputable brands will give you the same performance so choose a brand that is compatible with your reflectors. I just ordered the Galaxy Select a watt sinc I can turn it on the HQI setting and overdrive the bulb when it has reached it's four month intensity drop. if you want performance then M80 Magnetic ballast is the preference of a lot of the hardcore halide guys. Keep in mind that the M80's aren't nearly as efficient a electronic since they run at 396 watts so you're really making that bulb work. The result is a different coulour temperature, more PAR, but shorter bulb life. There is a good discussion about magnetic ballasts here
http://www.canreef.com/vbulletin/showthread.php?t=34315. Magnetic ballasts are getting harder to find since most everyone is going with electronic, I do know Hamilton still makes them and Aqua Medic makes a really cool Reflexx cube M80 ballast. If you decide on electronic, all of the sponsors here carry the mainstream brands.