My experiences (subjectively based on my own humble opinions...)
I currently have an A7s, EM1, (A7 mk ii on the way).
Previously I had and shot 5d mk iii, xe1, xt1, rx1r.
Rented A7, A7r etc...
I always regretted selling the rx1r, the sensor + optimised lens combo makes for an absolutely stunning IQ. The build of the camera body is gorgeous.
However the focus speed was slow and there were some ergonomic things about it that I didn't like for somebody with big hands - I'd prefer the X100/X100s/X100t in this regard. The IQ from the RX1R ruined me however. Until the A7 range and particularly the A7s for me, nothing came close in the 35mm FOV.
The Fuji's gave me a love hate affair. I loved the retro look of the camera's and the ergonomics took me back down nostalgia street. The Fujis were the cameras that I wanted to love. However I didn't like the build quality (mushy buttons on the back of the XT1, hollow feeling), nor did I like the IQ output from the files in Adobe LR. I'm still unconvinced about the X-Trans filter. Supposedly it solves issues with moire, I'm still unconvinced on that one. Effectively it's approximating each red green and blue value from a wider number of pixels, so resolution suffers a little and green detail in landscapes have been shown to hold less detail than 16mp bayer sensors. If you play with Iridient or PhotoNinja you will get significantly better results, but still there are 16mp APS-C and m43 sensors that I would take ahead purely from an IQ and workflow perspective. Nevermind more recent APS-C leaps from Samsung (NX1), Sony (A77 mk ii) and Nikon D7100 in that regard.
On the FF side - from a pure technical perspective, Sony has been leading the way from a pure stills sensor development. Sony has 3 A7 models - each will give you amazing IQ depending on your your shooting requirements and in different ways. There isn't one 'better IQ' than the other depending on what you want. Before the A7 mk ii was release I personally rated the A7s as the best all rounder for my shooting needs (good light and low light) hence why I picked one up - originally intending it as a video project tool. Since I picked it up I've discovered that it's a phenomenal stills camera and one that I am really really enjoying the results from. There is a very unique look that big fat pixels give you.
The A7r will give you D810 and medium format rivaling resolution. However shot discipline is important to actually realise that resolution potential and the shutter is very loud and clunky sounding. Personally speaking it's not for me currently - give it some IBIS, electronic first curtain and then that would make it a lot more interesting.
The A7mk 1 is a great camera and the new A7 mk 2 will probably produce the best real world IQ in a handheld FF camera in the most varied of shooting environments - IBIS has very tangible benefits on resolvable hand held IQ. Sony look to have fixed almost all complaints on the original A7/r line.
The A7s was released a number of months after the A7/A7r and is sort of an A7 mk 1.5 sitting somewhere between the A7/r and A7mk 2 line. Sony gave it faster AF, silent shutter, metal lens mount, better behaviour with legacy wide angle lenses (gapless micro lenses), different colour signature (now found in the A7 mk ii), improved AWB, higher quality video and codec options.
The A7s has massive dynamic range and is a more sensitive sensor than anything currently available in FF or smaller. However this will be at the expense of outright resolution.
If you shoot landscape an A7 or A7r will have 2 and 3 times the resolvable detail in really good light. That being said do not let 12mp put you off... I've printed 16X20 with the A7s and had lots of detail with no pixelation, certainly noticeably better than what I've gotten from a 16mp EM1 or from a 16mp XT1. Michael Reichmann has said that it is the closest FF camera he has come across in terms of having a medium format 'look' to it. Big fat pixels give a very unique special look. I tend to agree - the tonality from this sensor is extraordinary.
IMHO the Zeiss 35 2.8 is a fantastic piece of glass and is certainly very close in IQ output to the RX1R but you obviously lose a stop in speed. It's IQ is probably overshadowed by the 55 1.8. But that is almost as close as you will come to an Otus with AF. Perhaps expectations on the 35 2.8 are not as realistic as they should be.
If you don't mind manual focus, there is native 35 Loxia F2 which is probably the closest to the lens IQ as found in the rx1r. The results I've seen online from this glass is stunning and the lens is not that big either. Sony are also due to release an AF Zeiss 35 1.4 Distagon in March which could potentially be great. It will be huge so not a walk around lens, but likely in terms of IQ it *should* be the daddy of the 35mm's on the Sony platform.
On the Fuji side of the fence, Fuji's 23mm 1.4 is a phenomenal lens but again it's a large piece of glass. It's the best AF 35mm lens that I can think of in an APS-C or m43 body. This lens made it hard for me to sell the XT1 and I would enjoy it's results almost as much as the Zeiss 35 2.8 and RX1R.
Just my 0.02.