The difference is tremendous, it'll feel like your 5Dc didn't even have AF compared to the 5DIII. Low light performance is much better, and there are just so many little features that make it an awesome camera. On the same note, the 85LII is a friggin' awesome lens, stunningly sharp and the bokeh is out of the world. I'd say the only problem you're going to run into is having to send one of them back after the demo, since they are both awesome products.
But honestly I think you'll get more out of a new body than a new lens, the 5DIII is really a huge step up from the 5Dc. The 85 is nice, but I think you would end up using a new body much, much more.