Sure, I 'll give it a try. But do remember that I can only comment on the bodies/systems that I use. And, as I elaborate below, keep in mind the questions I initially asked above, as the answers to these will help when trying to understand the advantages/disadvantages of each system if you are in the market to switch/supplement/purchase new equipment.
m4/3rd's: I have now shot with Olympus and Panasonic bodies and lenses for about six years. I switched when I had health issues that prevented me from carrying my Nikon equipment. I thought the switch would be temporary, but I now use this gear as much, if not more than, my Nikon gear. The biggest advantages are the most obvious - size/weight/flexibility. Both companies make very compact bodies and lenses as well as premium gear where optics and quality take precedence over size. So, you can carry a basic kit in your pocket or you can pack a bag or pack with a bevy of bodies and lenses. The IQ, IMHO< is a step above 1" sensor cameras if you print large or shoot in lower light conditions, and is almost as comparable to APS-C cameras. And, if you pack a fast lens on a decent body, you can further negate some of the APS-C advantage without too much additional bulk. AF in normal lighting conditions on static subjects is pretty much instant (at least from most of the premium bodies/lenses). And if you have a good EVF, you get focus magnification and peaking when focusing. You can also easily see if an image is not clearly in focus, and this helps me when shooting close-up handheld photos. The lens selection is great and some premium Olympus bodies and lenses seem to do quite well in wet weather.
The two big downsides to this format are moving objects/C-AF and low light. Contrast AF systems keep trying to close the gap with respect to C-AF, but it is just not the same as a DSLR. Granted, a skilled photographer can bring home the money shot with either system, but beyond brand ambassadors, many who shoot action will probably not choose mirrorless like Olympus/Panasonic. Low light is also a challenge as there is no escaping the advantages of sensor size. Ye, I can shoot my E-M1 at ISO3200, but it is really at the limits, IMHO< and requires good NR techniques in PP. The "grain" structure of the noise is actually not bad on the E-M1, but it is still there. I try to limit the camera to ISO 1600 under normal circumstances when I can. I also need to mention that battery life is not the same with a mirrorless camera. It has gotten better, but not many bodies will give you more than 500 shots under normal conditions (and lots of action in a short period is not what I would call a normal condition). Think measuring in hours rather than shots.
APS-C: This is what I shot before using m4/3rd's (D300), and I just recently bought a D500, so it will be back in the line-up. My take on APS-C is that what you see, is what you get, mostly. It sits in the middle between FF and m4/3rd's, and IQ generally falls as such in comparisons. Two main things about this format. First, it can be found in both mirrorless and DSLR bodies, so really the comparison is not about sensor as much as it is about the body style. I am going to limit my comments to DSLR bodies as I have not shot with an APS-C mirrorless body, but I suspect that some of what I just said above will apply to those cameras. My take on APS-C cameras is that they serve two big markets. First, they are affordable (for entry-level bodies) for those wanting a DSLR. And second, and more importantly, they are a viable option for those who shoot subjects like wildlife and want the extra "reach". The D500, for example, give me many of the D5 features at a more affordable price point. And while it gives me more "reach" than the D5, it does not give me any more reach than the new D850, given the pixel pitch of the cameras. It is also an affordable way to get phase detection AF that generally give you better C-AF performance than contrast-based or hybrid systems.
Full Frame: At the cost of money (albeit less so these days) and size, you get all that a FF body can offer. A large sensor, shallow DOF if needed, and generally good low light performance. I have a D610 that I picked up form a friend a few years ago when he upgraded bodies, and it was an offer that I could not refuse. I like the camera for low light work, like nightclubs, or night shots, and the files seems to go on for ever and ever when I preview them for culling, but that is generally what you want. A lot of high quality pixels. While I like having this camera, I suspect that it is the least used of the bunch. Mostly because of size and the slightly uncomfortable grip. Still, having it available, especially for what it cost me, is great. As I said above, I like the right tool for the right job whenever possible. I'll wrap it up here as I could write numerous books on the issue. I am sure that others will have different experiences, and I would encourage you to take them under consideration as well before you make any decisions. And as always, YMMV.
Good luck,
--Ken
Who does not like a new toy?
