The electronic ballast should be more efficient and stay cooler because it uses square power pulses of variable width rather than an inductor to regulate current flow.
More things to go wrong, though.
If your electronic ballast goes bad...it could be weeks before it gets replaced or repaired. With a magnetic ballast, you simply replace the transformer/inductor and back running in less than an hour. Never a problem with which bulb to use any other problems associated with electronic ballasts. Just my 2 cents.
I prefer the digital electronic. Quiet and cooler. I can mount mine just about anywhere and not worry about burning down the house. You can also switch freely between MH and HPS without needing special conversion bulbs. If you have the cash Lumatek makes a sweet unit.