After a long review of both the format, here is the summary… Real world usage of Full Frame and m4/3
What I liked in D810 or FF sensor
Handling…some how it feels better than even D800E and other Nikon bodies I have used. Strength of sensor to pull back details from shadows, or recover highlights when mistakes are made in exposure, and a very clean image at ISO 64.
Something about capturing red color, like in brick or in dresses, which I felt Nikon did very well and accurately for reds.
What I liked in Pen-F or m4/3 sensor
Comparable or better image quality to FF for all my practical purpose, except with moving subjects. When exposed correctly can recover enough details form shadows or highlights.
The trick with Hi-Res mode, actually not for the resolution but much better color and contrast details, which I felt always lacked in digital sensor.
What not to like in both
Images above ISO 5000 in D810 ( probably due to the 24-120mm lens at f/4) I wouldn’t print them, and won’t go in albums for sharing. ISO 3200 is the one I am comfortable with the D810, and it us ISO1250 with Pen-F. If I have to choose above this ISO, I would just take picture from an iPhone, it would just be to capture that moment, not worth all the trouble of transferring, sorting and adjusting later.
Image stabilization with 24-120, gives a 2-3 stops at long end… I got reasonable results only at speeds faster than 1/30th of second. But, Pen-F gives sharp results at 1/8th of seconds for same view.
Indoor sports/action. Disappointed with both. Now I know why people spend $6500 for D5, Mark 1DX and upwards of $10,000 for lenses.