Bulbs are super easy going. With my dimmable 600w ballast I can run a 1kW bulb at 400,600,or 660, a 600w bulb at 660,600,400, or 350, a 400w bulb at 400 or 350. I can literally run any MH/HPS 400w bulb - 1kW bulb with it. Now of course you want to match the best you can and you don't want to go way over - it would NOT be good to run a 400w bulb at 600w.
I recommend the dimmable because you might have heat waves, you may have to move and run it in a 3x3 space, you might at some point want to veg with it... Say you want to veg with the MH bulb they give you - chances are your plants are small and don't take up a 4x4 area yet (or why are you vegging). Running a 1kW light 18hrs a day (time and a half compared to flower) costs a bit of money and if you only need to cover a 2x2 area you're wasting most of it. Now dimming it and running it at 400w over a 2x2 area makes a ton of sense. 1kW veg for 4 weeks is 504kWh or ~$60.00. A 400w veg for 4 weeks is 201kWh or ~$24.00. The savings of $36 should immediately offset the purchase of a 400w MH bulb (which you could use if the ballast was dimmable).
And of course the same principle applies to flowering - start them off closer together while they are smaller and run it at 600w before cranking up to 1kW when they are larger and more spaced out. After a few runs it should easily make up for the difference in price.
I just realized most 1kW ballasts only dim to 600w? You would have to change all the 400w to 600w but the ideas all still apply.
And you could always get the better ballast and cheap bulbs. A cheap 1kW bulb like the one that comes with the other package is ~$20. You can get a sylvania (which is a decent brand) 1kW bulb for $30. So for the extra $100 you could get the better ballast the same hood, and still buy both a 1kW MH and HPS bulb.
You would have to check and make sure these are the right ones but here's a for instance on the prices.
1kW Sylvania MH bulb $26 -
http://www.plantlightinghydroponics...-bt56-universal-metal-halide-lamp-p-1749.html
1kW Sylvania HPS bulb $29 -
http://www.lightbulbsurplus.com/index.php?main_page=product_info&products_id=398