Magnetic ballasts produce more heat and less light over time. I've experienced this first hand with a Hydrofarm 400 watt lighting system (remote ballasted) that is over a decade old.
The digital ballasts use a bit less power, put out more light, run cooler and are also quite. Digital ballast aren't as heavy as magnetic. I love that I can dim my bulb; I use this feature when plants are younger, when introducing new plants in general, when foliar feeding or if it ever got a bit too hot in the room... all I need to do is turn it down. So the dial-a-watt really allows you to control both light and heat output, because obviously the higher wattage you run the bulb at the more heat it will produce. When plants are young I don't have to raise my reflector so much, I can just dim the bulb.
I could also veg my plants under 250 watts, then start flowering at 275 watts... a couple weeks later give them the full 400 watts and for the last few weeks 440 watts; all under the same lighting system.
I think the bulb problems everybody is referring to mostly was due to incompatibility of some brands with the first digital ballasts on the market, and they've come a ways since then. My Lumatek also has a full 5 year warranty and I've been using it for about a year so far with no problems.
The only real advantage I see with magnetic ballasts is that you can replace specific parts within the ballast if it failed. They are also less expensive, but the digital ballasts really don't cost that much more... Well some places they do, but if you look around enough you should still find a good deal on a Lumatek.