Magnetic can run both MH and HPS too. for both digital and magnetic there are certain wattages that aren't switchable.
digital will be lighter and quieter and have to be completely scrapped if defective, where Magnetic can have the defective parts replaced. Also, look at the replacement warranty. some come with 2/3/5 years. The only one that i know of, that I'm thinking of buying, is the CAP Xtreme Ballast (magnetic and cheeper then Lumatek). They have a lifetime rebuild or replace warranty. All ballast have the ability to fail, so it just depends on what you need in features, what you expect to have around in the future and how much you can spend.