Bottom line: My clear preference for this legacy macro lens is the Olympus E-M5 body. This is not because of any difference in image quality, as they both do a great job. But the preference is because of what I believe is Olympus's better handling characteristics, at least when shooting macros.
I'll start my comments with the Sony NEX-6:
The NEX-6 has only a couple of potential advantages as I see it. First is "focus peaking" which helps the process of manually focusing a lens. But when shooting flowers up close it works inconsistently. I find it works well when focusing on the edges of petals. But not so much when trying to focus on the stamens of a flower. The petal edges usually have sharp edges against a blurry background. Here the focus peaking works. But mid-tone yellow stamens, for example, against a background of mid-tone green petals makes the peaking ineffective.
Another potential advantage is the larger 16 megapixel. The APS-C sized 16 megapixel sensor on the Sony is larger than the m43 16 megapixel sensor on the Olympus. (It is commonly believed that both sensors are made by Sony.)
A larger sensor usually translates into better high ISO image quality. I am usually shooting macros at a shutter speed of 1/200th and at F8 or F11, and this often requires higher ISOs (such as 1600). But to be honest, I just don't see a difference in image quality between the two cameras. Perhaps a difference would be noticed in very large prints, perhaps over 20", but with macros I don't think I've ever printed anything bigger than 12" on the long side.
Now let me move to the Olympus:
I own the accessory battery grip for the E-M5, and this helps immensely when shooting with the 105mm lens. That's because the lens with adapter weighs 21 ounces. With the grip, you can hold the camera body with all four fingers (not just three fingers like on the NEX-6, or the E-M5 minus the grip).
I also feel an extra level of camera stability with the E-M5 because the EVF is in the middle of the camera body, while it is on the left (rangefinder style) on the Sony. I realize that the center EVF causes me to press the camera against my nose, but this makes for a more stable platform. (Note: I have a big nose!)
And finally, with regard to stability, Olympus has an in-camera stabilization system that will work with any lens. Though I generally shoot macros at a safe shutter speed of about 1/200th second, so far I have left image stabilization "on". Don't forget to make the in-camera adjustment necessary to let it know the focal length of the legacy lens being used.
When shooting at apertures and shutter speeds that will generally require higher ISOs, I prefer to use Shutter Priority on both of these cameras with this lens. Because you are setting the F-stop with the ring on the lens barrel, setting the shutter speed in Shutter Priority essentially gives you Manual exposure control, in that neither the shutter speed nor the aperture will change unless you change them. For me, that is often 1/200th and F8.
Now, here is where Shutter Priority pays off. And that's because I have a preference for Auto ISO. I want to pick the shutter speed and aperture with this lens and then I'd like the camera to give me an accurate automatic exposure by adjusting the ISO accordingly. When it works well, I can move around the scene at different angles and the camera will give me a nice exposure by moving the ISO up or down as the light changes. The reason I wrote "when it works well" is because I believe both cameras have trouble with finding the correct exposure in shutter priority mode with old lenses. But this has been more the case with the Sony, as I find I nearly always need to use EV compensation, sometimes 2 or 3 stops.
EV Compensation in Shutter Priority:
First, with both cameras, and with legacy lenses attached, the Shutter Priority forces EV compensation to effect ISO only. It has no effect on shutter speed or aperture.
Unfortunately, adjusting EV compensation is not an easy process on the Sony. I have to hold the camera away from my body so I can look at the buttons. The EV button is the "down" button on the 4-way controller. This takes you to another screen without a histogram to watch. The new screen shows the EV choices in 1/3 stops. You then rotate the rear dial to add or subtract exposure. It will change the ISO, as I mentioned above, but you do not have a histogram to view while making the changes. (This I understand is not a problem on the NEX-7.) After inputting the EV compensation amount, you can half press the shutter button to get back to the original viewing screen. Only then will you see the effect on the histogram.
I much prefer adjusting EV compensation on the Olympus because you never need to take your eye away from the EVF because the adjustment dial is the front dial that surrounds the shutter button. As you turn the dial with your first finger, ISO is adjusted and you can watch the change right there on the histogram shown on the EVF. (That assumes you have the histogram activated in the menu... I always have mine on.)
The Advantage of Olympus's Smaller Sensor:For macros where you want more depth of field, the smaller Olympus sensor has an advantage. Using the same 105mm macro lens on the smaller-sensor Olympus and the larger-sensor Sony means that to similarly compose the same image, I have to stand a bit further back with the Olympus. This is because at the same focusing distance the Olympus sensor would only capture the center (approximately 3/4th) of any image that would be fully captured on the Sony sensor.
So how does this effect depth of field?
We usually think of the depth of field being determined by the F-stop. But it is also determined by the magnification. More magnification means less depth of field, while less magnification means more depth of field.
Because I need to be further from the subject when using the 105mm on the Olympus, this actually decreases the magnification and therefore increases the depth of field. This makes sense when you think that the smaller sensor in effect increased magnification (by cropping the outside of the image when compared with the larger sensor on the Sony) when focusing at the same distance, and the only way to make things "equal" is to compensate by decreasing magnification by moving further from the subject.
A specific example is that if I am shooting F8 on the Sony I can get the same (approx) depth of field on the Olympus using F5.6. Again, its because if I want to achieve the same composition on the Olympus viewfinder as on the Sony viewfinder, I have to be slightly further from the subject. Because increasing the focusing distance increases depth of field, to get an image equivalent to what the Sony captures, I must compensate by decreasing the depth of filed by way of providing a one-stop larger aperture (ex. use F8 on the Sony and F5.6 on the Olympus).
What then is the advantage?
The advantage is that the Olympus is letting in twice as much light at F5.6 as it would at F8. This then allows me to shoot at one-stop lower ISO (If the Sony needs ISO 1600 at F8, I can shoot the Olympus at ISO 800 at F11). All things being equal, lower ISO will result in a higher quality image!