I'm planning to buy a new camera body, and I'm unable to decide between the 6D and the newly announced 7D Mark II. Which camera should I go for? How much of a difference does a full frame camera (6D) make over an APS-C? I'm mainly into landscapes and a little bit of astrophotography. From what I understand, Full frames are much better for astrophotography, but is the difference in image quality big enough to justify the loss of all the other features? As far as I can see, the only advantage of the 6D is the full frame sensor. Every other feature that I need is much better in the 7D Mark II. Exactly how much of a difference is there in the image quality of these sensors?
From an image quality standpoint the 6D has a fairly significant advantage over any of Canon's current APS-C offerings. Since the 7D Mark II has been announced but not yet released and hasn't been in the hands of most reputable reviewers/independent testers yet, it is hard to judge the image quality. Suffice it to say it would need a totally revolutionary sensor that exceeds the image quality of any APS-C camera made by anyone now on the market to approach that of the 6D.
I can tell just by looking at one of my images whether it was shot with my 7D or with my 5DII or 5DIII. The advantages hold all the way through the processing workflow with regard to single images, so for landscape photos the 6D is definitely the more capable camera.
For astrophotography the 6D also has an advantage, but for different reasons. Since much astrophotography involves stacking multiple photos to create one image, the difference in noise performance can be pretty much equalized. But the difference in Field of View when using the same lenses can not. And the wider the lens, the more it usually costs and the slower it is. To get the FoV of a 24mm f/2.8 using a FF camera you would need a 15mm f/1.8 on an APS-C body. The EF 24mm f/2.8 runs about $600, There is no 15mm f/1.8, but an EF 14mm f/2.8 L II wil cost you about $2300.
The EOS 6D and the EOS 7d Mark II are very different cameras in so many ways making them hard to compare.
The EOS 6D is the least expensive full frame DSLR from Canon and the 7D Mark II is their most expensive APS-C. You say you are into landscape and astrophotography and in my world buying a 7D Mark II over a 6D for that is crazy. The 7D Mark Ii is made for fast moving action and has both an extremely advance AF system as well as high burst rate, none of them important for your applications. The full frame sensor of the 6D is to me more than enough to completely beat the 7D Mark II in landscaping and astro applications.
Full frame is not a requirement for the kind of photography you want to do and the APS-C alternative in this situation, to me, would be the entry level cameras. They are not nearly as expensive as the 7D Mark II as well a lot cheaper than the 6D.
It's especially hard to recommend a camera given that you have no experience with either full frame or APS-C and don't really know what you want from the cameras.
To just buy a $2k camera without knowing that is not something I would have done. I would borrow (or buy if I have to) an entry level DSLR and practice with it. When you know what limits your photography you know what camera you need. Don't spend thousands of dollar trying to figure it out.