Before you spend thousands of dollar on upgrading to a new camera system like FX from DX or from Canon to Nikon. I am not sure if you are using saved up cash or other means to support this venture. It's a big investment and making the right choice plays a bit part or you might regret it later and wishing you did not spend that money away. The manner which I read your message, how you wrote and trying to understand your rationale, observation or motivation to upgrade, I get the impression, you are still relatively new or have not really taken photography really seriously or have not spend enough time with what you own enough to understand the fundamental and appreciate what your camera really can do or what other cameras can do depending on what you pay at various price point for each model of cameras.
To some extend you are comparing Apple to Oranges. Not about brands but DX and FX formats. Within each format, differences in quality and options changes depending on the price point.of each successive models. But even then it can be incrementally better. When you compare your own DX DSLR to the FX you borrowed, of course you are going to see a mark improvement. Without trying a similar price point FX from Nikon, you are misguided to think the Canon is better all because it's FX or it's Canon. That is so far from true heheh.
Before you spend a lot of hard earn cash, good to see what others will advice but watch out for white noise in the advice's too. As not all views are neutral. That is to be expected in a forum. I say, go further beyond CS for answers or helpful tips. From your photos I can see you should be good enough to take test shots yourself to evaluate. Invest a few dollars renting something of equal quality and standard to the Canon 6D since you like that quality but from Nikon's range ( Like the D610) to see what you think of colors, resolution and functionality...etc Try also using your existing DX lens on the FX body and switch the DSLR to DX crop mode. FX bodys cost more and FX Lens are just as pricey for the good ones. So you might want to have your DX around to use for a while if you need to buy your gear in parts.
You can rent so easily these day to do your own testing. Also don;t just shoot in one color mode. You do know you can set a few variation of color space mode like more saturation, more skin tone..etc. Some time you might actually be on the wrong color setting that is why something your friend set on his 6D now seem to look so wonderful heh But in actual fact, your own camera could have gotten you the same colors as what you see in the 6D or allow you to tweak close to it.
We can tell you stuff till we are blue in the face but they would reflect our personal scope of knowledge, preferences and yes even BIAS. It means nothing if you are not convinced and possibly ending up more confused with you buying something you realized is not for you. Or you bought it but hardly can justify the higher price for your new system since you seldom use it. heh. Heart break leh to see something so good collecting dust and dropping in value as it age
YouTube have so many good videos on photography, have lots of technical, product reviews and tutorial guides introducing you to sensor formats pro and cons, resolution, color spaces depending on camera brand ...etc. Waiting in CS for someone to write may or may not help you extensively. Especially if we use technical terms alien to you, the more you read, the more confuse you get heh. YouTube are visual videos. Covering subject from basic to advance on even buying tips. Just be sure to read more then a few reviews of the same camera gear so you can work out what are bias marketing talk and what are facts.
You need to understand fundamental of not just shooting but get some good idea about camera gear. Without that, you are not going to come to a firm or correct choice for you. Yes paying the most money will get you the best quality shot (but to a point too depending on also on your experience skill but if you are not serious about photography, you are not going to make it worth your while 'over' paying for a gear once the excitement of owning a new toy is past the honeymoon period.
As you are willing to invest even more money into the hobby so must you invest a lot more time and effort to getting better at shooting and knowing the equipment. If you buy a good camera and you put it to PROGRAM MODE... I say, best you stick to what you already own.
