a 250watt ballast, you may need 2 ballasts, 1 for HPS & 1 for MH unless you get 1 that can run both HPS/MH
Correct. Or, you can get one bulb as a conversion bulb, which means it will work off the ballas of the other kind. (in other words, a MH conversion bulb will work off a HPS ballast, and vice versa). FYI, the only difference between a ballast for MH and for HPS is that the HPS ballast has an igniter and the MH ballast does not. A switchable ballast lets you turn the igniter on or off depending on the type of bulb you have.
Digital ballast, Lumatek or Nextgen. Makes things much easier, switchable (can use both MH and HPS) and more light. What more could you ask for?
True, and digital ballasts run quieter and cooler as well. But, to the question of "What more could you ask for?", how about a lower price? A digital ballast can be quite a bit more than magnetic.
$149 isn't too much for a 400w unless you are on a strict budget. but yeah, it is a little pricey if you are a small timer.
You need to know that the bulbs will need to be replaced after a year. If you're looking down the road money wise you need to consider that. What may be "convenient" now could be costly long term. That said I would get a switchable ballast and replacement bulbs when you need them at Lowe's or Home Depot