there are a few differences between digital and old school magnetic ballasts.
1. digital ballasts are more efficient. they use less energy and put out more light
2. digital ballasts don't hum like old school ballasts so they're stealthier
3. at least when they first came out, digital ballasts weren't considered too reliable, but lately growers are reporting they're better
and 4. digital ballasts cost more. i DID find really cheap digitals that were about half the price of the big name units, but i wonder how reliable they are.
i can't make any promises for reliability, but i think HID hut has some of the cheapest ballasts out there $80 each for 150w HPS & 175w MH! at that price you could buy two and have a spare, but do they blow up in a week?
http://www.hidhut.com/catalog/discou...1e1eec9b68a491