Definitely the 6D. Together with a 16-35mm wide-angle lens you are perfectly equipped for this task.

I use both cameras and would use the full-frame 6D without hesitation for the purpose you describe. As a lens, the 24-105mm (with its still-just-a-bit-telephoto) or the 16-35/f4 wide-angle zoom, which is enormously versatile because of its stabiliser.

Especially when it comes to architectural photography, the answer can only be full format. Especially when minor shooting errors such as tilting and the like have to be corrected, you need reserves for cropping.

Clearly ,EOS 6D with the right lens

I have switched to full format (80D was also the alternative). I am satisfied, especially in low-light conditions (compared to my old 550D). It does have downsides though: There is less choice of affordable lenses, they are more expensive, heavier and the range of use is more limited compared to EF-S. I have the EF 24-105 L and 75-300, but I considered this when I bought it. I don't have a wide angle (yet).