night and day actually. Digi is THE way to go if you can afford them, and they are coming down in price all the time.
The biggest differences are: no noise at all, utterly silent. they don't slowly degrade over time like magnetics, they are more efficient, allot of the models are dimmable (400 4 veg, 600 flower), and the most important difference is the frequency that the light is refreshed. Digital ballasts send juice to the bulb much faster than magnetic ballasts, the electricity is actually "pulsed" to the bulb from the ballast. Think of the refresh rate on your computer. This is why digi's claim 30% more light. Its not more lumens per say, but 30% less time that the bulb is off. Look at people's pictures.. see some with the horizontal bars in the photo.. magnetic ballasts.