Well, since I have Leica FF (M-P), Leica APS-C (X), and Olympus mFT (E-M1 and E-PL7), I find that all three have their strengths and weaknesses as cameras ... and all three produce superb image quality.
I use the E-M1 as my "technical, must get the shot" camera. It's fast, outrageously full-featured, solid as a rock, and there are some amazing lenses for it. It's a great format for longish FoV zoom lenses as they can be much smaller and lighter than the same FoV for APS-C or FF. The image stabilization is amazing too ... It's so much easier to hand-hold the the E-M1 with an effective 300mm FoV at f/5.6 than it is to hand-hold the Sony A7 or Leica M-P with a 180mm f/4 fitted.
As I said before, there are a bazillion ways that work...
G