It depends on your taste, what you're looking for in such a lens, etc. You may want to do what I did and rent one - which may prompt you, as it did me, to buy one. Nice though the 85 1.8 is on a Rebel (I've used that precise combination), on your 6D the 135L will provide better bokeh and shallower focus (among other things), not to mention better low light/high ISO performance, even though the fov will be much the same. (I'm surprised, though, to see some say that the 135 focuses faster than the 85 1.8 - my 85 1.8 focuses extremely fast as, I thought, did everyone else's. Could they be confusing them with the 85L?)
Note, too, that it's not just a one-way street - try the 135L on your Rebel too, where it will give you 216mm fov equiv. (albeit at a slight cost in some other respects), while, as you've doubtless found out, the 85 1.8 performs better on your 6D than most (all?) 50mm lenses do on your Rebel.