The 6 Plus should have better CPU performance (higher clocked), but worse GPU performance (more pixels to push).
I think that is not a breaking point, there are a little difference, all the way iPhone 6 plus is better than iPhone 6.
I chose the 6+ because of the higher PPI and resolution, better camera, and improved landscape mode. The 6 feels like nothing more than an overclocked 5s. I feel there is little reason for its existence.