From what I saw of the UK preorders at least, the 32GB went up in date first, before 16GB.
Which is to be expected with the new Retina display people know it will use more, and people who had 16 before are upgrading to the next size up. 32 will be more populer or the same if anything.
And I am pretty certain even if the 16 did sell out first it would make no difference, people would just get the 32 instead.
Wheras if the 32 sold out not many people would downgrade to the 16 instead.