Pixl and MDBT42Q deep sleep power figures?

I don't really know if that makes a difference. Pull-down (so 1 when pressed) is probably better because ideally you want as much stuff grounded as possible when in idle.

However I think on the nRF52, having pins set as inputs means that whenver the pin changes there is some electrical activity inside the chip which at least partially pulls it out of sleep mode. Basically as long as the pin isn't 'floating' you'll be fine.