The 5D3 doesn't need interchangeable focussing screens.
You can have a choice of grids showing up in the OVF, and I use the coarse one all the time.
It's really good to have.

I reckon the 6D sounds really good.
It's got some great features that even the 5D3 doesn't have like inbuilt GPS and wireless file transmission, and full time auto focussing for video.
Give it 6 months or so, and the price will drop below $2K, and I reckon it wil be a great seller.

In my 5D3, I use a Lexar 1000X CF card and a Lexar 600X SD card (90MB/Second), and I find this SD card to be fast enough for almost anything, and it's faster than any 2 year old CF card, so there's nothing wrong with newer SD cards as far as speed or capacity goes.
I think the 6D will appeal to those moving up from crop sensor DSLR's and about the only thing against it would be that these people will either have to have EF lenses, or get rid of their EF-s lenses and trade up.

It'll be interesting to see a comparison of the Nikon 600 and the Canon 6D in the near future.