I´d wait until the first reviews for the 6D come but it seems like the focus is reaally good in low light. Thats why i preordered mine.
I dont mind the SD Card Slot, besides i would have liked 2. It supports uhs-1 which means you can get cards that are 95Mb/s. Thats plenty fast for every user.
What would you want to get for the 400 bucks? in all honestly 400 bucks are nice, but in the world of photography you dont come very far with 400 bucks. You could buy a nifty 50, which isnt great for video, and a 85, which is an awesome lens.
If you really would want some primes for video, again the m42 lenses+24-105 for stills (with the 5d2).
Are you able to afford a 6d+Tamron 24-70vc? You would have 2.8, VC in a great lens, and (i hope) a great body.