Another aspect to consider when picking a system is accessories other than lenses (if you use any). Canon and Nikon SLRs have every flash, remote, extension tube, bellows or any other accessory you might possibly want, including stuff made for very specialized applications. Especially Nikon has kept the same basic lens mount for more than half a century, so if you want a gizmo to hook a Nikon up to essentially anything you might want to attach a camera to (ironically, other than a wrong-mount lens, due to Nikon's deep flange distance) it probably exists. Canon's selection is almost, but not quite, as deep. Remote triggers can be more expensive for the locking sockets on these cameras than they are for the headphone jacks and USB ports everyone else favors (although there are now cheap knock-offs).
Every other system has its own mix of what is and isn't available. Some are fairly comprehensive, while others have glaring omissions. It's not necessarily related to how good the lens system is, either. I would give Fuji credit for the best lens lineup outside of the two full-frame titans (Micro 43 close behind, but I think Fuji takes it by a bit due to the overall high quality of their line). On the other hand, Fuji seems never to have heard of a flash! They offer two rebranded (and overpriced) cheap Sunpaks, one flash of unknown origin (maybe even actually built by Fuji) that is better built, but radically overpriced for its limited capabilities, and they are supposedly introducing a higher-end flash that nobody has actually seen.
Other than flash, Fuji does a pretty decent, if slightly random job on accessories. Replacement lens hoods are, for some reason, hard to buy, sold as spare parts rather than accessories; but they make their own custom Arca plates of very high quality... I'm not as familiar with the foibles of other makers, since Fuji is where I've ended up settling. As far as I can tell, everybody has their ups, downs and occasional battery shortages!
DSLRs other than the smallest models have a notable advantage over all mirrorless cameras other than the Panasonic GH4 and (oddly) a couple high-end Nikon 1 models. DSLRs use much higher capacity batteries, which are somewhat larger and heavier, but get many more shots than the minuscule mirrorless batteries. Since mirrorless cameras are always in Live View, they may burn through batteries faster as well - I've never had the opportunity to shoot the two with comparably-sized batteries side by side. Apart from the GH4 (and perhaps those Nikon 1s), don't expect more than 300 shots from any mirrorless battery! Sonys seem to have especially bad battery life and notably slow chargers, but their battery meters are accurate and report percentages . Fuji battery meters are like Subaru gas gauges - from half a tank to empty in the blink of an eye. I own quite a number of batteries and always have a spare - fortunately, they're light.
By the way, I agree that Micro 43 is fine for anything up to 13x19 (easily, on the modern bodies), unless you have a very high dynamic range scene.
