The sensible option is to remain with my crop-inspired lenses and maybe go to a 70D or the next Rebel, but the 6D seems pretty cool.
There are mainly two questions you need to answer. First, and most important; how much money do you like to spend on a tool you do not need to make a living out of. I think you know this, since the 70D and the 6D are not so far appart on the price scale.
The second qustion is, what you like to do with the camera and the file; are you an allround shooter, or do specialize in something; do you need/like to make large prints, or do you almost only make smaller prints.
FF (the 6D) will give an advantage in IQ; you will see this especially at high ISO and in large prints. With smaller prints and at lower ISO the difference is rather small. On the screen APS-C pictures can look quite nasty when zommed to 100% (my 70D files at high ISO do so), but at 20MP the resolution is so high, that it does not really matter in normal prints.
On the other hand, the 70D has a very nice AF-system, a cool touch screen (this is no toy, it really improves the handling), the higher frame rate, and other nice features.
If you are a landscape or architecture photograph, then I would go for the 6D. The IQ is great, but it is not a "fast" camera. The 70D is more of an allround package, with a little disadvantage in IQ; which is okay when you do not need large prints.