Well, I'm by no means an expert, so I should probably just keep my trap shut. But what I lack in experience, I make up for in enthusiasm. So here goes:
Obviously, if HPS is an option for you, defintely use it over the flos. In my opinion HPS is quite acceptable for vegatative growth and unsurpassed for flowering. (Except, of course, by the Sun.)
I've never heard of "Fat *******", but I assume it has that name 'cause of the bud characteristics. In which case, you really ought to use HPS for the flowering phase. With a name like that, it would be a shame to use flos for budding.
Simple equation. More engergy (light) in = More mass (bud) out. Sure, it's *possible* to cook a hamburger with a bic lighter, but wouldn't you rather use a campfire?
Better cool off that attic with a fan, too.