They're two very different cameras aimed at very different uses. If its moving fast, getting the frame at the exact right moment and in perfect focus are the most important factors to look for in a body. If its stationary, timing and focus aren't difficult, so sensor performance is the most important factor to look for in a body.
The rumours for the 7D mk II suggest it will be more of the same - crop, very fast frame rate, very fast/good AF with great tracking, and for something which isn't a 1 series camera, very solid build. In other words, if you have crop glass that you want to use, or you want a 'cheap' body which can shoot action well (in other words, something in the ball park of a 1D X/5D3 in terms of AF/FPS, but not IQ) , then the 7D series is a much better buy than the 6D.
But if you're prepared to skimp on everything that makes the 7D series great in the world of crop cameras for single frame picture quality (slower/static subjects), then the 6D with its full frame sensor is a much better bet (even if Canon come out with a new killer APS-C sensor, this generation of FF will be better).
If your car shots are static and you're not tied to EF-S glass, I'd recommend the 6D.