I am going to do my best to explaine the difference between the 2 types of ballasts so that most people can understand.

First a standard ballast is just a transformer that takes your line voltage from the wall and converts it to say 10,000 volts at 60hz. A transformer does this by what is called electromagnetic induction, so in other words while the transformer is running it is creating a very large Magnetic field, in doing so it is expending energy in the form of heat. that is why a standard ballast gets hot. So in reality a 600w ballast is actually using around 700w of energy.

In a digital ballast this voltage conversion is done with electronic rectifiers. So what is happening is the rectifiers are just not converting the voltage they are taking your wall voltage and converting it to say 10,000 volts DC at 20,000hz, by doing this it saves the energy of the magnetic feild needed in standered ballasts plus by running the bulb at 20,000hz and using direct current it will prolong the life of your bulb.

Now for the down side of digital ballasts, they are a solid state device and can be damaged by excessive heat, that is why they have fans in them. They can also be damaged by power spikes and surges where as stanard ballasts cannot as easily. If you have a digital ballast make sure you place it where there is plenty of free air flow, do not put them say in a box or enclosed space. Also put them on a surge protector just like you would put a computer for the exact same reason.
In my opinion digital is the way to go.

Happy Growing,