Based on the videos I've seen so far from the 6D, I'd say the extra $1,000 or so is worth it to avoid moire/aliasing. The 6D seems to have no better video quality than the 5D Mark II, especially with regard to moire/aliasing. Add to that the ability to monitor audio and I would say that the 5D3 is worth it, assuming you have the budget. Given the moire/aliasing on the 6D, then I think the key question for video is between the 5D Mark II and the 6D, if you can live with moire/aliasing. Is the 6D worth the extra few hundred? Having said that, when I used a 5D Mark II, it felt its age. Video was great. It just felt old in how it operated.
Look for deals that come up on e-bay or others from autorized retailers, as the 5D3 price is now reliably often in the $2800-$2900 range.
Personally, I'm looking forward to the 7D replacement. I would love to have a crop-sensor body that doesn't have moire/aliasing issues and has improved low-light capability compared to the current crop sensor cams. While I like the 5D3 (the one I use is actually my wife's) - especially its almost nonexistent moire/aliasing and its low-light capability - I have found that I think I might actually prefer the crop sensor for video. The DoF is plenty shallow - and not too shallow, like the 5D3 can be sometimes. I actually still use my 600D/T3i quite a bit, even when my wife would let me use the 5D3.
If moire and aliasing don't bother you on your current 7D and the lack of headphone output isn't an issue for you, then I would say to get the 6D (or 5D Mark II, if you can find one) and spend extra on glass.