i give another vote for a digital.. i have a galaxy dimmable digital ballast that goes from 400 watts to 250, and simply love it.. i can run the 250 for veg and bring it up to 400 for flowering..
it's also very nice to be able to run both mh's and hps bulbs off of one ballast imo...
they even have some ballasts that you can run them at full power, ie 100%, then turn it down all the way to like 300 or so i think it is..