What is the logic of different card slots?

It appears that the sd cards are more adept and cheap for video use ,but to be honest , nikon had to really push it with the D4  even resorting to XQD card to get the high frame rate .

I get the feeling thiat if the D400 is to achieve anything near the same frame rate as the D300 ,with a 24mp sensor ,it too will have XQD. even a bigger faster buffer wont cut it alone ,and if the XQD  cards are not to go the way of the dodo ,then nikon need to use it on more than just one model, that, and a recent announcement from Lexar ,that they intent to be producing them by the end of the year . leads me to think its a real possibility for the D400.