They are two very different lenses...
14mm is insanely wide on a full frame camera like the 6D and can be difficult to learn to shoot with. It has rather strong "moustache" distortion in the center of the image. That can be corrected in post-processing, if need be. It may not be an issue for landscape/aurora shots. But would be for architecture, etc. The Rokinon/Samyang lens is manual focus and manual aperture, so will be slower to work with. That's likly no problem.
35mm is sort of a "slightly wide" standard lens on a FF camera like the 6D. Little distortion. The original 35/2 is a compact, somewhat lightly built lens without USM. It has a lot of fans and looks a little like the cheap 50/1.8 II, except the 35/2 has a metal bayonet mount. The new 35/2 IS USM seems a lot better built and to have improved IQ. It's larger and heavier than the original, just out this year and sometimes can be a bit hard to find. Of course, it costs more than the original 35/2, too. The original EF 35/2 sells for around $280 US, while the new 35/2 IS USM is currently offered for $550 US. The lens hood isn't included with either lens, the original EF 35/2's OEM hood sells for about $23. The new 35/2 IS USM's hood is a ridiculously expensive $54.
Personally, for landscapes I'd probably go for the 14mm. Be aware though that unless they are really filling the night sky auroras and other distant objects might be rendered too small and appear too far away in your images. An ultrawide like this exaggerates perspective relationships a lot... "expands" them, so to speak... where a 35mm focal length renders perspective more true to life unless you get super close to the subject.
BTW, the Samyang sells under many different brand names and all are the same lens. You'll find it as a Rokinon (who is probably the actual manufacturer), Bower, ProOptic and more. There's even a Vivitar version, though they label it a 13mm. It's exactly the same lens, though, so shop around. Also, if you buy one test it immediately and look for sharpness across the whole image. If one side appears softer than the other, there might be a problem with a decentered element. This seems to be a quality control issue that shows up occasionally with the 14mm lens (in any of the brands mentioned).