If it is up for a vote, I'm backing mixed card slots. SD cards might not be the fastest or most secure, but they are universally available. Every supermarket, tourist attraction and smaller camera shooter carries SD cards. And they're cheap. Plus nearly every laptop and tablet has a built in SD card reader. Mixed card slots give you the CF card slot for speed and reliability and an SD card slot for those "OMG I forgot spare memory cards" moments.
Besides, I thought we were almost at the point where memory card speed wasn't having a significant impact on camera FPS or buffers?