D600 Vs 6d...Wide-Field Astrophotography

I know this subject has been beat to death (6d vs d600) but I haven't seen anything specific to Wide Field Astrophotography (will not be using a tracking mount so need to keep exposures short via iso\aperture).  You can see examples of the type of stuff I'm trying to do on my 500px link in my signature.

I've compared the two extensively but am having trouble understanding which has better high ISO performance (raw) at 3200\6400 and better shadow performance for something like Astrophotography.

At the same time I'd likely be using it 50% traditional landscape\50% nighttime wide field astro stuff...so I don't want to give up anything either way.

I have no lenses from either Nikon or Canon so I'll be starting fresh, I've played with both shortly and I have to say I like the handling of the 6d a bit better. AF is irrelevant for me as this will be mostly for Landscape and because I'm coming from a X100 (so both will feel amazingly fast  ).  I've made a similar inquiry on the Canon side...sell me!

Thanks for any input.

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http://500px.com/Joe-K