Actually, I was going to mention the A7s, but then you get into does the OP crop or ever print big, etc, etc, and it sounded like they liked their current lens line up, and if budget is an issue, I've looked at switching so many times (including the other day, as the bodies are so cheap) then by the time you start buying native lenses, it goes back to sticking to what you got is probably more economical, unless there is a huge change in what/how the op is shooting. So, yes, kudos to the A7s, but the 6D may get them where they want to go, and if that doesn't, then certainly a 5DIII probably will, and the 7D2 *might* get there, depending on if someone spends the time learning that post processing skill set to maximize what you can do.
Could also do a dual body situation, stick with the 7D until light requires them to switch to the 6D - and most lenses will go on both bodys. All cameras are a trade off of something anyways.
Op, it goes back to your budget - what can you spend, and are you willing to get that better low light image quality and have to stick with a center point to get it if you are looking at 1000 or under for a body. Or, you have to save up for a 7D2 and learn better post processing, or save more and go for a 5D3 so you have the AF you want and better low light. No right or wrong, just choices.
Godox/Flashpoint r2 system, plus some canon stuff.