Om-D vs GH3 for stills image quality, which is better?

The better photographer. The cameras are equal.

I own a GH2 and it is only marginally less then the OMD/GH3. If 100 pictures I have taken, don´t work out as I hoped only one or two would have turned out a little better with a better camera. The rest was either not such a good idea to start with or pilot error. And both do really happen. Some situations don´t lend themselves for a good picture and a lot of my failed shots I can see what I should have done differently afterwards (and I might even remember that next time).

If I had to choose between them, I would go with the best ergonomics.

You could also go for best look and lighter....