Flex is what you want - no need to insulate if you're pulling from inside the room. If you're worried about efficiency of the air exchange use two fans/carbon filters and two seperate runs of ducting. The non-flex is almost too efficient and it is such a headache to install. I personally don't like to do runs of more than 5 lights on one fan, you'll either start seeing inconsist temps from one light to the next or you'll pull too much air over the bulbs making them too cool to operate at peak efficiency.
Exhausting your room air through your hoods is the only way I would run sealed hoods, but you need to make sure your hoods are SEALED if efficiency and/or smell are an issue. don't trust the cheap rubber/foam grommet thing in the hood, seal them with foil tape and seal your ducting with foil tape too otherwise negative pressure from the fan will be sucking unfiltered air through cracks in the hoods (it's a great way to waste C02 too).