is it goose or duck down? and which is better?

Heat transfer as it relates to sleeping bags happens by two mechanisms, convection and conduction, with convection being the more important of the two. Andrew's point about sleeping bags utilizing only air not being efficient is valid because insulation comprised entirely of a liquid or gas which moves easily will subsequently redistribute heat easily. On the other hand, air does tend to have a lower conductivity value so it is useful in reducing heat loss via conduction. I don't know if the material down and feathers are made of has a high or low conductivity value, but the critical usefulness of these is in allowing air space while simultaneously minimizing air flow. A sleeping bag utilizing only air as an insulator would work much better if the air inside never moved. So, yes you want dead air space, in a sense, its just a bit more complex than that.

I ended up thumbs-upping woodenpickle because I had knee-jerk thumbs-downed it but can't just take it back and remain neutral. I would say woodenpickle is both correct and incorrect. Yes loft is the "dead air", but I think there is benefit to those air pockets being smaller rather than large. To take it to the extreme opposite of a wet bag that compresses all the loft out, if maximum loft vs insulation was best then we'd all just roll in inflatable sleeping bags (all air, no insulation). The Goatfolk must be referring to a sweet spot where the finer consistency of the goose down over the duck down allows for the loft and yet breaks that loft up in to smaller pockets of air that are insulated by the little walls of goos feathers.....something like that anyway.

Wow, I don't know where that information came from, but the whole point of insulation in a sleeping bag is "dead" air space. Then the heat from your body warms the air trapped between the insulation fibers. That's just how sleeping bags work. That's why they always talk about loft. You want loft.. It creates air space that you can then heat up. Hence the reason down doesn't insulate when it's wet.. It all sticks together (loses it's loft) and there is no air space to warm up.

gray goose down, 800 fill power. goose is the only 800 fill. duck can only be about 700 fill power.

Its mostly a matter of preference..but generally Goose down is finer than duck and so it will be warmer, less dead air space to hold cold air.