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.
When you look at it, one thing is clear. The difference between the two is not image quality but all the specs around it. Sure the 5D has more focus points and an extra card slot which are great detail upgrades but is it really worth the mark up in price? Personally I don't think so. Unless I'm high rolling and want to burn 3500 I'll stick with what I got and make it look like a hasselblad lol.
And what about the 1DX? We'll if you need the ultra fast shutter speed, which is not much needed outside of sports photography then really it's just something to make you look cool. But again if your want all those details outside of image quality then go for it.
But in the end what does it really matter? I've seen people shoot amazing photographs with rebels and other aps c sensor cameras. So bigger is not always better especially when you've only been in the game for 2 to 3 years. Obviously an upgrade is always nice but remember to make what's in front of the camera better quality than what your shooting with. Gear is gear at the end of the day and nobody can match the person shooting what's in front of them. So first spend your money on what will be great picture apart form a 6000$ camera and use your creative imagination. No camera can beat that!