At all apertures the Olympus lens was slightly lighter and had a shutter speed that was a stop more. As you can see the Olympus result is lighter, though not by a stop. I did the test twice just to make sure I hadn't done anything wrong. It was the same both times.
Now this isn't that unusual and I have seen the same thing before, when comparing lenses. Often between lenses where on one aperture is set manually and the other electronically. Why there should be this discrepancy, I cannot say. As you see the images are photographing the same thing, and I see nothing that would affect the shutter speed.
In terms of which I prefer, the Nikon lens gives the result I'm happiest with, in terms of exposure.
Just one of the reasons I like to check out the lenses I use. So I don't get any "surprises" when I use them for real.