The Canon side holds an advantage in quite a few lenses -- and especially in the most commonly used lenses like the 70-200mm f/2.8 and 24-70mm f/2.8.
I looked at the difference in dynamic range and, to me, it's just not enough to be compelling given what else you'd have to give up. People seem to cite DxO as a source for this, but DxO doesn't show you "data", they show you "scores" and they don't reveal how they come up with those "scores". Every time I check a different site that DOES show real data (and sample images) the differences are (a) not significant and (b) trade places at higher ISOs. Given that DxO won't reveal data, show sample images, and seem to get conflicting results from everyone else who will show data and sample images, I find that DxO lacks credibility.
The Canon 6D has an AF working range of EV -3 to 18. The Nikon D610 has an AF working range of EV -1 to 19. So the 6D can lock focus in merely 1/4 of the light that the D610 requires.
For astrophotography there's a much bigger gap. The Canon does exposure simulation on the liveview screen... the Nikon does not. Focusing a DSLR through a telescope is usually done through the liveview screen so this is a really big obstacle. If you try to focus, say... the Moon... you'll see a white "blob" on a black field and nothing you can do to focus will reveal any contrastable details that will help you focus. (The new Nikon D810a corrects this -- that camera has exposure simulation in liveview mode so that you can focus through a telescope using liveview, but it's unique to that camera's firmware. Hopefully Nikon puts this in every camera. Canon and Sony have had it for years.) You'll also find that every astrophotography application that can control the camera for image acquisition supports Canon. Only a few support Nikon (and almost nothing supports Sony, Pentax, Olympus, etc.) It's just never been a popular platform for astrophotography. The only way I managed to get the Nikon to focus through the telescope was by using a Bahtinov focusing mask and a bright star... but even that was a challenge because the software that performs the focus didn't support Nikon. I ended up having to take a deliberately over-exposed image, inspecting the diffraction spikes created by the mask, adjusting focus and taking another test shot, and repeating this numerous times until I thought the focus was close. It was very difficult and time consuming (but it can be done.)
The folks who make Backyard EOS (probably the most popular image acquisition software for astrophotography using a DSLR) have finally released Backyard NIKON (I haven't seen it yet, but they advertise that it's available.) These apps are about $50 and probably worth every penny. If you do choose to go Nikon, you will definitely want something that can help you with focus (Backyard EOS has a built-in focus-assist that uses the "full width / half-max" method (FWHM)). FWHM works well if you have exceptionally stable skies. If not, the slight "wobble" that a star does due to atmospheric instability (the stuff that causes stars to "twinkle" -- astronomers call it "atmospheric scintillation") will create the illusion that the star occupies more pixels ... tricking the focus software into thinking that it is not focused. This makes it a challenge to focus using FWHM if the atmospheric "seeing" conditions aren't good. A Bahtinov focusing mask over-comes that problem (a physical device you put on the front of your scope (like a lens cap) that has special slits in it and it can help you focus.)
I generally don't think the camera platforms have a very significant difference... your own skill, composition, lighting, technique, etc. will easily be THE factor that determines how well your images turn out. I think people get a little wrapped up on the technical side, amplify specific details (and forget to consider other details), etc. For normal usage, I doubt a case can really be made that any major brand is truly "better". It's hard to buy a "bad" camera these days. That's why, when I see people talk about dynamic range, I feel that frankly all major brands are entirely lacking. When you need dynamic range you'll be using HDR or reaching for your Gradient ND filters regardless of which brand you use (even the Nikon guys).
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