At least my take on this (and I ended up landing on Fuji for GREAT bodies and lenses, both in quality and in a wide lens lineup - the lenses aren't cheap, but they're about the best around).
It absolutely DOES depend on print size, up to a point... Remember that very large prints are viewed from a distance - Nikon's 4 MP D2h was used to shoot billboards - you see a billboard from 100 meters or more away, while driving past. (For that matter, only the very most modern stadium scoreboards are even 1920x1080 HD, despite being 50 feet tall - the only reason a scoreboard looks at all reasonable is because you can't get close to it).
The largest print size I deal with routinely is 24x36" (I own an Epson 7900, and there is no place nearby with a 9900, so finding anything bigger is a real pain - I could do it in a special circumstance, but it wouldn't be easy). So for me, a 24x36 that stands up to inspection on a table before it gets framed and hung is what I'm after. This will differ among photographers, of course.
I hike (planning to thru-hike the Appalachian Trail in 2017), and a non-trivial percentage of my landscape images are in the backcountry, so small and light is a relatively high priority (as is sturdy/weathersealed) - even if I could afford a Phase One system, it would only work for 50% of my needs, because it couldn't GET TO the other half. I am willing to have two partially overlapping sets of lenses (some lenses that are good as well as being light, and some that are great but heavier).
Canon or Nikon DSLRs tend to be either too flimsy (D3300 and friends) or somewhat heavy. Pentax, with their lighter, weathersealed bodies, is kind of intriguing, but I worry about their staying power.
Sony E-mount (not FE - see below) has only extremely low-end bodies (despite terrific sensors), and is short on lenses, with only a couple of good ones plus the better FE lenses, which are huge on the little bodies.
Sony FE-mount seems extremely appealing (a LOT of resolution in a very portable package), but I don't fully trust the sealing or build quality, and the lenses are limited in range, large and some of them are not stellar (others are).
Micro 43 is very appealing from a body size (and durability, especially in the case of some of the indestructable Olympus bodies) standpoint, and there are some really nice lenses (mixed in with a lot of junk), but I just don't quite trust the resolution or dynamic range. Unless there's dynamic range trouble, I can get a GREAT 12x18 out of Micro 43, and I can generally go to 16x24 except on very high detail images. I won't go to 24x36, unless the image is a foggy, atmospheric shot anyway.
Fuji finally came out with a weathersealed body, and I've always loved their lenses. Their seems to be about one print size of difference between 16 MP X-Trans II and Micro 43 - 16x24 is EASY from a Fuji, and 24x36 is possible on a lot of subjects. I have a big deposit down on the first X-Pro 2 to reach Vermont, because that last little boost (24 MP instead of 16, and a couple of sensor generations newer) should put the X-Pro 2 firmly in the 24x36 class.
My equipment saga will almost certainly at least pause at an X-Pro 2, an X-T2 and a bag full of Fujinons. I'll take whatever of this gear makes sense on a given shoot, but I'll know that I have access to focal lengths from 10 to 400 mm, all at uniformly high quality, plus some very compact lenses for long hikes. I'm not going to carry both bodies at once on the Appalachian Trail, but I'll probably switch bodies a few times over the six months, so I have access to both sets of features and ways of seeing.
For complete lines of high-quality lenses, there are only three choices - Nikon FF, Canon FF and Fuji (and Fuji is only complete if you don't need exotic telephotos or tilt/shift lenses). It's pretty well possible to get to a complete setup in Micro 43 as well, but it takes more dodging and choosing. Neither Nikon or Canon have sufficient options in their crop lineups, except by augmenting with full-frame lenses that are heavier and sometimes odd focal lengths. None of Sony's FOUR sets of lenses are near complete by my definition (the A-mount lines come closer than E or FE-mount).
One interesting new consideration with digital (especially mirrorless) is that some mounts are easier to design lenses for than others. SLRs are more forgiving, because any mount that avoids hitting the mirror also has sufficient distance behind the mount (flange focal distance) for most designs. Film is more forgiving, because it'll accept light from any angle, while digital sensors need more nearly parallel rays. The FE mount is apparently brutal to design lenses for, because it has an extremely short flange focal distance for the size of the sensor. The only lenses that will work with that constraint, especially when corner quality is taken into consideration, are large, heavy retrofocus designs. The Leica SL mount is going to have the same problem - its flange focal distance is equally short. This issue doesn't seem to plague APS-C and smaller mirrorless designs, perhaps because the flange focal distance is not as short in relation to the size of the sensor (Micro 43 actually has a LONGER flange distance than Sony FE, by a tiny bit).
If Sony FE is "undermounted" - its APS-C origins mean that the mount isn't really deep enough, Micro 43 may be "overmounted"? The flange focal distance is relatively long for the sensor size, longer than Fuji, Sony or Leica's APS-C mount (the latter two of which have full-frame variants), and the mount diameter is only 6mm smaller than the full-frame Nikon F mount (and even closer to a couple of obscure 35mm mounts).
If I were Nikon or Canon, and considering getting into FF mirrorless, I have a radical suggestion - use your existing mount! Yes, there's going to be a bulge in the middle of the camera to get an F-mount in there, but it's still possible to save a ton of weight by ditching the prism and some of the grip (as well as, in Nikon's case, body focusing motors and related hardware - restricting it to newer lenses may well be a perfectly acceptable tradeoff for weight savings). A D810 is about 12 ounces heavier than an A7rII (and a D610 is only 8 ounces heavier). Some of that is battery, and Nikon should probably keep the larger battery - almost every A7rII shooter carries a bunch of spares anyway.
The prism is a large, heavy component, and it should be possible to save 6 ounces or more by replacing it with an EVF (the mirror and all of its associated hardware come along with the prism). Trim the grip down a bit, partially by going to the D610's dual SD slots instead of the D810 SD+CF configuration. Somewhere in the 24-25 ounce range (the A7rII is just over 22 ounces), it should be possible to have a mirrorless equivalent of a D810, with a fully functioning F-mount allowing massive lens choice. You can get the weight back in one basic lens... Nikon's 24-85 f3.5-4.5 is slightly lighter than Sony's 24-70 f4 Zeiss, and has a useful boost at the top end. More extremely, Nikon's 50mm f1.8 is 4 ounces lighter than Sony's 55mm f1.8. Most of the other primes are lighter for Nikon as well, although FF wide primes are getting heavier in general, because digital sensors work best with the heavier retrofocus designs.
The real weight savings come between FF lenses (whether SLR or mirrorless) and dedicated APS-C lenses.Fuji's 18-135 "travel lens" weighs 17 ounces, Sony's 24-240 (most people would say it's not as good a lens) weighs 30 ounces. Fuji's 56mm f1.2 portrait lens weighs 15 ounces, while a Nikkor 85mm f1.4 is 21 ounces. Fuji's 10-24 ultrawide zoom is 15 ounces and Nikon's 16-35 is 24 ounces. Fuji's 16-55 f2.8 is 23 ounces, while Nikon's portly 24-70 f2.8 is nearly 40 ounces.
If you can put up with the smaller sensor's image quality, Micro 43 lenses are significantly lighter still. The superb Olympus 12-40 f2.8 PRO is 13.5 ounces, more than half a pound lighter than the Fuji, and 1/3 THE WEIGHT of the Nikkor, although the other two Olympus Pro lenses are much closer in weight to their Fuji equivalents.
Every photographer's choices about how much weight they're willing to carry for increased image quality will differ - Ansel Adams got 8x10 inch view cameras surprisingly far into the backcountry (although many of his most famous images were made from the top of his car - he knew the weather and locations in Yosemite incredibly well, and made obvious locations special - Clearing Winter Storm is from a National Park Service visitor parking lot, but the timing was incredible).
I chose an intermediate solution, while there are certainly justifiable reasons for going both larger and smaller. An advantage which Fuji shares only with Micro 43, Canon Ff and Nikon FF is that the cameras, lenses and mount are all designed for the same frame size, and to work together. Sony's FE mount is an APS-C mount lightly modified for FF, and that has consequences for lens design. Both Canon's and Nikon's APS-C systems were originally FF, and only limited lens lines (heavily focused on cheap kit zooms) take advantage of the smaller size. Once you get beyond the limited APS-C lenses, you are using unnecessarily large lenses with odd focal lengths (until you get into telephotos, where coverage doesn't mattar - any 300 mm lens will cover medium format, so a Micro 43 lens will not be smaller than a FF lens of the same focal length and maximum aperture (it will, of course, have a smaller angle of view).