Are you able to test-ride either? I think that electric shifting is incredibly cool, but have to admit that I've never tried it. Some people seem to love it and claim that they'd never go back to mechanical; other people dislike it and claim that they prefer mechanical. I assume that I'd really like electric shifting, and for that reason I'd prioritize the bicycle with electric shifting over the disc brakes. And from a practical standpoint, unless you're riding in an area with many stop signs and traffic lights, it's likely that you'll shift more than you brake, so... unless you're either cool to or actively dislike the electric system, it makes sense to spring for that.

Another way to look at things is to assess the cost of aftermarket upgrades. If both disc brakes and the Ultegra Di2 shifting system are important to you, how much would it cost to add disc brakes to the 2013 model vs. adding the Ultegra Di2 to the 2015 model? I'd imagine that the former would be cheaper but I admit that I have not priced out either.