Here's THAT question again - Micro Four Thirds VS APS-C ?

hey all, I've been using a 2011 Nikon D5100 DSLR for a few years, a hand me down from a family member.  I almost never end up bringing it around as often as i should , not only because of its bigger size, but also the autofocus is pretty frustrating on it, to the point where i've actually sharpened (no pun intended) my manual focus abilities, and tend to jump to that primarily from the start.  It just would be nice to have a camera with wicked fast autofocus from the latter half of the decade, and I keep hearing so many cameras supposedly promise such precision.My style of photography is mainly of low-light, non-flash people photography, to which I've absolutely adored using a 50mm  f/1.8 prime lens.I want to make a switch to a more pocketable camera. I've been very curious about APS-C and Micro-Four Thirds cameras.  However, it just seems like not one particular camera is  100% the right camera for me.  I see so many pros/cons to all of them.  I would love to have your feedback especially if anyone wants to add to or debunk the common complaints of each brand.On the Micro Four Thirds tip i've been leaning towards an Olympus EmD Mark 2, but have heard that Olympuses in general have a bit of an issue with indoor exposure (usually underexposing) and high ISO noise (prob cus of the lower sensor).  I also hear capturing depth of field  doesnt come as easily as with full-frame . Can anyone confirm this?On the APS-C tip, I'm leaning towards the Fuji XT-100 for its price, but I hear its a bit lagging in the autofocus precision, at least compared to higher Fuji cameras in the next tier (XT2, XT20 etc).  I've also been very curious about Panasonic's line of cameras, as one of my favourite cameras i've ever owned and still use to this day is their 2008 Lumix  LX-3.I've also been very curious about Sonys A-6000's line, but am more gun-shy about them as i've heard / almost as many people singing its praises about being hte undisputed champion of sorts regarding its autofocus .... and yet being undeniably THE most prone series of camera to overheating.If anyone wants to suggest other cameras, add their own experiences, or can verify / debunk my findings on some of these cameras, please let me know and I would love to hear it / discuss!

Well, let's see if we can straighten you out on a few things.

I used a D5100 for 4 years, and with good lenses it works absolutely brilliantly. For me, that meant FX lenses. Not one DX lens. I got rid of the kit 18-55 right away.

Auto-focus performance is a product of both the body and the lens. You can find dogs on any body.

The D5100 is APS-C, just like the Sony A6xxx and that Fuji. The micro-4/3 is smaller, but manufacturers are doing amazing things.

The Sony overheating issue was for video, I think. Not image capture. Apples and oranges. Yes, every goes on and on about Sony sensor performance in low light.

Olympus and exposure: deal with it. You'll learn how to use the camera and compensate. I don't see a problem here.

Panasonic continues to make good products.

If you do some searching here on the forum you'll find plenty of threads discussing micro-4/3 / mirrorless systems, and lots of opinions. I encourage you to do your research, not just ask one question in one location.

The best low-light performance? A bigger sensor. You might consider the a7, or wait for the forthcoming Nikon mirrorless.

I used Nikon for many years before switching to Olympus. I currently have the E-M1MKII. I have never had a problem with underexposure on any Olympus camera, indoors or out. Exposure is exposure no matter where you are. If you pixel peep at higher ISO you will see noise in the images. In reality, if you print the images, the noise goes away. The images will look smooth as silk. The IBIS is the MKII is amazing. I was fooling around yesterday and played with a 2 second hand held exposure. It blew my mind with how sharp it was. The big advantage for m43 is the size of the lenses. My 75-300 which has an equivalent FOV of a full frame 150-300 is about the size of an 24-120 full frame and is lighter. The system isn't for everyone, but it works for me. Look up any of my images to see if the quality is up to your expectations. 
Now the Sony. Like Gary said, the overheating on the A6500 is primarily in video although I have seen a couple of claims with overheating for stills. I disagree with Gary's recommendation of the A7 because you say you are frustrated by your Nikon's Focus. The A7 has improved with firmware but is still a dog for focus except in very good light. The new A7III is a different story. I had it for a while and the focus is very good. I really like the camera, but the lenses are very expensive. It's a major investment.
If you want to stay Nikon, there are many later bodies out there that are excellent. The D7200 is very good and it's a little older. You mentioned DOF with full frame. Yes, it's easier to get a shallow DOF with full frame, but it can be done with any format. Shallow DOF seems to be all the rage right now. It has it's place, but it's not for every shot. Mostly portraits. A lot of times we struggle to get more DOF especially in landscapes. It all depends on your style.

I am also intrigued by the Fuji X-T100 body. But the new kit lens sounds like a dog.

For the insanely fussy, I have a few Fuji trials on my blog. Here is a link to one -

photographyintro.com/fujifilm-ae-program/

Kelly Cook