Becoming a more serious photographer, we all want to step it up when the time calls. So full frame is definitely the way to go. I'm a canon shooter so I'll stick to what I know for now just so I appear like I know what I'm talking about. But really when It comes to buying a new camera why I choose the 6D was very simple. The price and it's quality.
What's the difference between a 5D and 6D? After looking up a bunch or articles and watching a ton of videos, to be honest it's not all that much. So the price difference can be a huge deal breaker with choosing the two.
Here's a nice chart laying out the difference between the two.