As far as using a combination of SD and CD cards for different cameras; that should not be a problem if you are at all logistically competent. In fact, IMO, using two different types of cards is, IMO, easier than shooting with two different types of batteries such as when I travel with my 40D and 7D. In addition to different batteries, I need to bring two different types of chargers. No great problem but, just another piece of gear to travel with.
I would prefer that the 6D was not so totally bare bones! I still have my first digital camera; an Olympus C5050Z which surprises me in the sophistication of this camera which I purchased sometime in the 1990's. The little Olympus was capable of using both CF and SD cards. I seems to me that this would be a rather easy add on....
The SD is really too bare bones for me but, if I were a traveling photographer, IMO embedding GPS coordinates is might be kind of nice, however it would not really be not really necessary. OTOH, having the capability of adding a voice annotation to my images would be just great. I work around this by using the video capability of my 7D as a note taking accessory.