I'm no MH ballast expert, but here are a few things:
Electronic ballasts are generally more efficient (use less juice) than magnetic ballasts.
Electronic ballasts give of less heat than their magnetic counterparts.
Electronic ballasts are more expensive than magnetic ballasts.
Different ballasts of the same power output drive the same bulbs differently, so for a given bulb certain ballasts work better than others by putting out more PAR or giving off a slightly different coloration.