The Nexus 6 will likely release with a Snapdragon 801 or 805, following the trend of the recent smart phones.
If it launches with a Snapdragon 801 it will most likely stick to a 1080p display, however if it uses a Snapdragon 805 it will have a 2K display.
Considering that the performance of Snapdragon 800/801 on a 1080p display is identical to the 805 on a 2K display, it really depends on whether you are willing to wait for the extra resolution.
The Tegra K1 isn't possible at least not now for mobile phones, it generates too much heat and could possibly also eat the battery pretty quickly.
I went with a Nexus 5 nearly two months ago instead of waiting for a Nexus 6, so far I have loved it.