By dual heater cores, do you mean two seperate cores, or an individual dual-pass heater core?
As for the temps, not too big of a difference, but big enough to notice. I'd only suggest those smaller rads if you have major space restraints. If you use two of those small rads, you'll still get good temps.
As for placement of rads, I think you could get away with putting a large one on the floor of the case, exiting downwards. With that setup you'd have to have the pump in cages behind the intake fans. Otherwise, if you can relocate your HDD's to the 5.25" bays, you could put a heater core in front with dual 120mm's - it'd be a tight fit though.