I shoot with both a 60D (soon to be upgraded to a 7D2) and a 6D, with different uses for each camera.
60D: used for birding with a 400mm f/5.6L no-IS, benefiting from the crop factor (more pixels on the bird), although you will hear a lot of argument in the birding community about FF vs crop. However, to get a FF action body is expensive. 60D is also my one camera and one lens (EF-S 15-85mm) general use / travel / "casual" camera. I started shooting more landscapes and especially night landscapes and stars, and the high ISO performance of the 60D was wanting compared with the 6D. There is about two stops difference in the amount of noise: 60D ISO400 (minimal noise) = 6D ISO 1600 (minimal noise), 60D ISO 1600 (significant chrominance noise) = 6D ISO 6400 (significant chrominance noise). This makes a huge difference if you are shooting landscapes with stars, because reducing the chrominance noise in post processing also dampens down the star color (yes, they have many colors). I also wanted to play more with ultrawide angle lenses and do more narrow-depth-of-field shooting, both of which are easier to do on full frame. So that's why I ended up with 2 cameras. As it turned out, I started playing with vintage manual focus lenses on adapters because I had some good film era lenses and I didn't have a lot of full frame-capable modern lenses in normal and short telephoto lengths. My 50ish and 100mm lenses are old AIS manual Nikkors - pretty good optically - I have to pixel peep to see the chromatic aberrations stopped down one stop.
I had been shooting with the 60D for 3 years before I decided I wanted FF for the above reasons. I had accumulated a number of specialty lenses (400 f/5.6, a macro, an ultra-wide, a fast normal lens 35mm f/1.4) in addition to that great utility zoom 15-85. I have had 24" x 36" prints made from the non-cropped 60D files, and they look perfectly fine at 4 to 6 feet away, a normal viewing distance for a print that size. For close viewing, I have printed 60D files at home up to 11" x 14" with fine results. There's no question that the low-light 6D files have more subtlety in color and less noise, but if you shoot in adequate light levels, you can't really see a significant difference at small print size.
For most people, a good tripod and head, some off camera flash, and an additional lens or two would give you more options than spending the same on a FF body.