What resolution should we use for iPhone 6+ development?

I’m came here a little confused and I hope someone can answer my question.
I’ve read here that on iPhone 6+ the effective resolution is downsampled by hardware from the exact 3x 1242px by 2208px to 1080p. My question is whether Unity build apps will get downsampled as well or do we need to use an exact 1080p graphics?

The phone does the downsampling, so you should design at the actual resolution used 1242px by 2208px. Good article to read here: The Curious Case of iPhone 6+ 1080p Display | by Bruce Wang | We are appcepted | Medium

It is rendered at 1920x1080px, not 2208x1242px due to performance reason.

Source : http://forum.unity3d.com/threads/iphone-6-plus-screen-width.269264/