There's a LOT of size tolerance just under the surface of questions like "Df vs XT1" and there are absolutely no right or wrong answers. Modern full frame sensors are better than modern APS sensors. Hard to say exactly how much (depends on which ones you're comparing) it's roughly speaking about a stop or a bit more than a stop. Whether the larger size of the Df is worth that extra stop or so over the Fuji sensor is an individual call. I'm wrestling with that right now - I'm OK with the size of the Df body but I'm only OK with it if I limit myself to relatively small, cheap, and slower prime lenses. Faster full frame lenses, at least in the DSLR world, are much larger and much more expensive than Fuji's excellent lens lineup. And I'm not willing to either pay for or carry those lenses with the Df. Whether a full frame DSLR is even something I'd call a serious compact is a highly debatable point. It's an awesome camera and my tolerance for it is higher than I thought it would be, but it's the biggest camera I've ever shot with, so that's a real consideration.
I think lens size / performance / cost ultimately become where the tradeoffs live. Smaller full frame bodies are clearly possible as Sony is demonstrating with the A7. But the lenses then get a lot trickier. Sony's 35mm f2.8 is pretty small but it's only a pretty good lens (unlike the larger exceptional one on the RX1, much of which is buried inside the camera body). The 55 f1.8 seems to be exceptional and reasonable sized, but it's getting pretty long, and 55mm isn't exactly the most challenging focal length to pull off a fast lens. Whether Sony can come up with good, fast, smallish lenses when they start playing around the edges of longer and wider focal lengths is a big question. Fuji seems to be showing you can built QUALITY fast glass for relatively reasonable amounts for APS. They're not small lenses like m43, but they're not bad either, easily handleable. They seem willing to sacrifice some on size for quality, but APS lets you do a lot more with a given size than full frame. Look at the 56mm f1.2 that Fuji's bringing out - it's not a small lens, but it looks pretty well handled on the XT1. Imagine what a similar 85 f1.2 or 1.4 would look like on the Df. I've played with a Nikon 85 f1.8 and it's not too bad, but it's pretty much at the far end of my personal tolerance range for Df lenses. Similarly, Fuji's 23mm f1.4 is a great lens at a reasonable price. In the full frame DSLR world, you can do quite well with a 35mm f2.0, but the attempts to get to f1.4 then get very expensive and quite a bit larger. Similar at 28mm where I'm using a Nikon at f2.8 and the Fuji's comparable 18mm is similarly priced at f2.0 and is nearly a pancake. Neither are stellar lenses, but both are really nice working lenses for those who don't spend too much time pixel peeping the corners wide open. And to get to a notably faster and better 28mm for the Nikon, both the size and price go up quite a bit. Never mind the telephoto and ultra-wide lenses where full frame get huge, APS get's pretty big, and m43 starts looking better and better.
So, on balance, to stay within my size and price considerations, I'm buying lenses about a stop or more slower for the Df than I would be with Fuji. Depth of field is close enough for my purposes and so I get back the stop or so from the sensor with lens selection in the Fuji world. So where I'd be shooting at 12,800 on the Df, I'm shooting at 6400 on one of the Fujis with a one stop faster lens. And realistically, as nice as it is to have, I don't shoot at 12,800 much on the Df and I wouldn't shoot at 6400 very much with the fastest Fuji glass. I'd probably shoot a lot more at 6400 on the Nikon and 3200 on the Fuji, which are pretty close from a quality standpoint.
So, obviously I'm wrestling with this a lot at the moment because I've sold my RX1 and I'm shooting with a Df that I'm contemplating buying. But I'm also thinking about jumping back into Fuji instead because their lens lineup has really matured lately and I could envision a kit I'd be really happy with, keeping the EM1 around for focal lengths longer than 90mm and maybe the ultra-wide end. The XT1 is a notably smaller body than the Df. I'm fine with the Df's body size, but that doesn't mean I might not prefer something smaller. And the bottom line is that sensors are likely to continue to improve, probably a lot. But the laws of physics that govern lens design are only gonna be bent so far. So there's a chance I go back to APS and m43 instead of full frame and m43. I don't know yet, but it's a real question.
So, no, the Fuji sensor doesn't stack up to the Df, but the system as a whole may very well. There are a lot of tradeoffs involved. I'd say Fuji's sensor, even at the ripe old age of two years, is still pulling it's weight. I'm not one of those big time Fuji guys who think it's better than any other APS sensor, but I think it's roughly as good as any, and that's pretty damn good.
-Ray
Fuji needs complex, very fast and costly lenses in design and make to compensate for the smaller sensor. If one is only after comparable Dof control, the optically very good Nikon D line, 1.8/50, 1.8/85 ought to be compared to the fastest Fuji glas rather than equally fast Nikon lenses. They cost a fraction of the Fuji 1.4/35 and 1.2/56mm and as far as the Nikon 85 versus Fuji 56 mm are concerned weight less. Lastly, none of the Fuji primes is weathersealed, which to me, is a flawed produt strategy. And the wethersealed new zooms are bulky and heavy. If one does not mind MF, the compact Nikon 28 ais is excellent, beating Fujis 18mm`s optical performance.
Besides specs and measurable differences there is an intangible "otherness" between all the cameras I used, including the XPro1, A7 and others (and as you know there were/are too many) and the Df. The latter feels simply more "competent", sturdy, reliable and responsive than MILCs. Imho Fuji`s X-Trans sensor IQ is overhyped. At lower iso its detail rendering is lacking and the high iso performance, again, goes at the expense of details and color accuracy. In the last two years Fuji seems to have studied the tale of "The Emperor`s new Clothes" (for its 16MP sensor) and made it their product strategy rather than improving on its promising, yet not fully delivering sensor technology. The only Fuji product, I eventually did not regret buying in the last 3 years, were the X100 (what a great camera shaking up the industry) and the 1.4/35mm and 18-55mm lenses. What I really like about Fuji: They continue to develop and build AF lenses with aperture rings and some even with distance scales. And they work on FW upgrades for products now longer made. A little rant from my side.