Me personally... I am a DIY guy. So any money I can save on a part will go for another part in the grow room. So me... I would buy the cheapest ballast I could and get a deal on the bulb, so I can afford some more nutrients and a new pH probe for my meter. (insert whatever you could use)
I guess the digi's give off a little more light, and give off less heat, and take a little less power to run... but like DST said, I have more than enough bud, my grow room is cool for the most part... actually could use a little more heat (yes, nice problem to have), and I think the ballast would save me around $2-4 a month on electricity, but that's just a guess.
As for bulb, I have to say I just got a Hortilux Eye, and man that give's off some nice light. You can totally see the HPS and MH spectrum being kicked out. They are expensive bulbs, but I found one new on Craigslist for $50. No box... I think I can deal.
Good luck my friend.