I think in your situation, fluorescent lights all the way through might be the way to go. It's true that HPS performs better in flowering, but that's not to say fluorescents are incapable of it. Take a look at this thread from another forum if you have any doubts, I'm sorry if this isn't encouraged (linking to other forums) but I think this may be helpful:
hxxp://www.thcfarmer.com/forums/f54/insane-membrane-1142/
As you can see, it takes more work with fluorescent, he had to use tons of LST to get the results he did... But in his situation, with his lack of ventilation, fluorescents were necessary and he made them work. Good luck with whatever you decide to do, but especially with your first grow, and it being bag seed, I wouldn't invest in an HPS/MH just yet.
Last thing, if you do get an HID setup 600W HPS lights are the most efficient lumens per watt, I'd get one of those instead of any other setup.
400W = 55,000 lumens, 137.5 lumens per watt
600W = 95,000 lumens, 158.3 lumens per watt
1000W = 140,000 lumens, 140 lumens per watt
Those are all general numbers and similar bulbs, and different bulbs can get different outputs, but for the most part the 600W gets the most bang for your buck.