This is a riff on the discussion taking place in the thread called "4/3 vs Nikon APS-C."
I have both full frame and m4/3 systems (and a Sony RX100compact and a cell phone camera) and I've found that the handier the camera is, the more you tend to give up in terms of control.
But to limit the discussion to interchangeable lens cameras, I've found that discussion tends to revolve around four aspects of these various-sized cameras: physical size (dimensions and weight), lens speed, depth of field aspects and sensor qualities.
The advantages and disadvantages of physical size is pretty obvious to all of us, and I think is really a matter of personal preference, a balancing of all the different ways we may shoot as individuals. Your best system isn't mine, simply because we shoot in different ways and may have different subject interests.
It's figuring out the other aspects that's tough.
I would suggest (as I did in the earlier thread) that lens speed (at least in terms of a one-stop difference) is not as important as it was with film, because digital ISO abilities are so much better. In film, I really felt handicapped in terms of quality even shooting at ISO800. I think digital has a two- or three-stop advantage here, which means that an f4 lens may practically be as good as an f2.8 used to be.
Except for depth of field. Here there are two aspects. You have less depth of field control with a smaller sensor (like a m4/3 compared to FF) for any given lens speed. But is depth of field control really important in general purpose lenses? (This is an actual question of mine, not a rhetorical one.) How often do we want to limit depth of field in general shooting, and how much depth of field control do we practically have with f2.8 zoom lenses on even a FF camera? Isn't depth of field control really something you want to deal with in specific circumstances, for which an even faster, more specialized lens would be more effective? The practical effect of this question comes from the other thread, where somebody pointed out that depth of field with an m4/3 system's f2/8 is more like the equivalent of f5.6 of a full-frame system, while a FF's f4 is the f4 we're all accustomed to. But is depth of field really a matter of interest in choosing between f4 and f5.6-equivalent?
Then there are pixel questions. A lot of high quality cameras seem to be settling on offerings in the mid-20s (24-26mp.) The next Panny GX8 will have ~20mp. People have argued since the dawn of digital about the storage capacity of larger vs. smaller pixels, and also about the printing abilities of different numbers of pixels. Is there really a discernible difference between ~20mp and ~25mp at common, fairly large (say full-page large format magazine) sizes? Do marginally larger individual pixel sizes make more difference than the camera's processing engines that produce the final image?
In the guitar world, some people obsess about the sounds produced by vintage, usually incredibly expensive (now) instruments like the '59 Les Paul and the older Strats and so on, but a friend of mine pointed out that given all the different skill levels, by the time you run the signal through a couple of stomp boxes and maybe a looper and out an amp, and given all the different acoustic qualities you might have in any given room, you're sometimes lucky you can tell that it's a guitar being played, much less a quarter-million-dollar 59 LP. Are the analogous qualities true of cameras? That by the time you finish running a scene through your personal technique (tripod, no tripod), the autofocus, the lens glass, the sensors, Lightroom, and finally the printer...does it really matter much whether it's an m4/3 or a Nikon D800?
