Only you can decide if the price difference is worth it. But the 85 on crop is like 135/2.8 on FF (in terms of DOF, noise and FOV) with much worse sharpness/CA/PF wide open:
Those crops do not say it all: the 135L has excellent bokeh.
Wow!!! The results from the link above tells it all. Thank you.
On the other hand, the 85 should perform quite well on your 6D. Some find 135mm a bit too long.
Make sure you're comparing with the same camera and aperture. The embedded link compares the 135 on a FF body at f/2.8 to a 85 on a crop body at f/1.8. No contest.
If you compare both at f/2 with the 1Ds III, then the 85 f/1.8 looks much better than it did on the crop but still falls short of the 135 f/2.