I've been very pleased with my 6D, which I've owned for around 4 years now; it consistently produces good looking photos and has very good low-light high ISO performance. It's also quite small and light for a full frame DSLR. For the money they fetch on the used market these days it must make them something of a bargain. I certainly wouldn't pay more to change it for a 5D III, which is an older design, unless I desperately needed more AF points and focus speed, which is unlikely for landscape work. The only thing that would currently tempt me to upgrade my 6D would be a 5D IV. However, we're all different and I think perhaps the best way to decide would be to try both cameras in both good and challenging lighting conditions and compare the results, both in terms of image quality and user experience, handling and portability.