Depends on the type of video you are planning to shoot with it.
I use it to record short clips of my kids and on a day out with the family, that type of stuff.
For this it works really good, and I can easily switch to photo mode (so I only have to carry one device for photo and video)
I often use 1728 x 724 x 25fps which gives 17 seconds of recording time per clip (after that it needs to write the full buffer (250mb) to the SD card, which also takes about 6 seconds writing time (250mb/40mb/s=6 seconds).
So you got a blackout period of 6 seconds after each clip because of writing down the buffer to the memory card.
Continuous recording resolution on the 6D is 1280 x 720 x 24/25fps. which still is not bad, but once you are used to the higher resolutions, 1280 x 720 sucks... 
If you are planning to shoot commercials, weddings, short films etc.
You should definately go for the 5d MK II, only because of the compact flash, which gives you better continuous recording resolution.
Although the 5d can not handle exFat formatted cards, I don't know how it behaves after the 4GB file limit while recording (does it write a second video file without missing a single frame ??)
EDIT: If you're also going to use it a lot for photography, then go for the 6d. (Newer sensor, less read noise, no pattern noise, better dynamic range etc.)